It happened like this: slowly.
Just like everything else in their relationship, beginning with that fateful day five years ago, when he'd convinced her they'd be Mulder & Scully (whoever they were).
Slowly, they learned to get along.
Slowly, they learned to like one another.
Slowly, they learned to protect one another, no matter the cost.
Slowly, they fell in love.
Even after he woke up with the knowledge that he loved her, the evolution of their relationship still moved at a snail's pace. He didn't know if she felt the same way; she didn't know if he knew the hell she'd gone through during his coma. So they let things happen on their own, with no pushing, no prodding, no rushing. After all, why hurry when what you were moving towards was forever (or the foreseeable future, which was far more reasonable and logical than forever)?
It became a dance at first; a welcome-home hug filled with excitement that lasted a little bit longer than all the other hugs. Two admissions of love, though they were qualified with a statement of pseudo-professionalism (and therefore didn't truly count). They began spending more time together - more than ever before. Now, instead of allowing cases to initiate and dictate their time together, they began meeting for breakfast at the diner every morning. They were already having lunch together at least a couple of times a week, and meeting with everyone at the Founding Fathers for drinks after work most Friday nights. Then there was dinner. Aside from the Friday night drinks, one would usually call the other (perhaps on a Tuesday or Wednesday) and suggest take-out or the diner or the bar, and they sat together and shared stories of their workday, or their co-workers, but mostly of themselves.
She had learned during her one attempt at dating his boss that she simply couldn't do it anymore, biological imperatives be damned. She couldn't put that look on his face, or that roiling pain in her stomach again. She knew exactly why she'd done it in the first place, but apparently she'd lost some of her ability to compartmentalize, and she found she could no longer lock away her feelings in a little box and push it into the recesses of her mind. Every time she saw him, they tried to escape. And when she'd tried to properly seal that box by dating someone else, they seemed to take on a life of their own and sprang forth, stronger and more intense than ever.
The next step in the dance came in the form of an almost-kiss. Both were quite certain that had the Squints not interrupted them, they would have closed those last few inches and felt the soft, warm press of lips against their own. But the moment was lost at the sound of their friends' voices, and the rest of the evening was filled with speeches and drinks and dancing, and no further opportunities to have a moment alone to finish what they'd started. The dancing was the best they could do under the circumstances, yet he had a hard time holding onto her for more than one song at a time. After all, as the one responsible for the new discoveries regarding Anok and Meti, she was an extremely popular person at the event. He felt it inappropriate to refuse the polite requests from other men, simply so he could keep her to himself. Instead, he whispered a witty remark about the requestor's appearance, winked, and left her to it. He took a turn around the floor with both Cam and Angela, but quickly returned to her side the moment he saw an opening. He was sure he saw her eyes light up each time he approached. Whether it was in relief at being saved from her current conversation, or in delight at having him close again, he didn't care; as long as that light never disappeared.
They left the exhibit opening the moment they saw an opportunity, choosing instead to order hot chocolate at the diner: cocktail dress, tuxedo and all. He thought they were the best-dressed diner patrons in all of history; she thought perhaps that was an exaggeration and adjusted the assessment to the past twelve months - a far more realistic statement, she said.
After that, they each began invading one another's space a little more often, a little more easily. Hands grasped in a show of comfort and support; walking down the street with a mere hair's breadth of distance between them, allowing shoulders to bump and hands to brush as they swung their arms; her reaching for and pulling off his ties with little thought of his personal space or how that single act might make him feel (in the biological imperative sense, of course).
And then at Christmas dinner, after everyone was done celebrating, while her dad and second cousin were still in the kitchen washing the last few dishes, he said goodbye and leaned forward, placing a lingering kiss on her cheek. She seemed to be glowing when he pulled back, and after wishing him a Merry Christmas she opened the door and saw him out, waiting until the elevator doors slid open and he'd stepped inside before slowly pushing her apartment door closed.
It wasn't until the third time he did it that he realized it was becoming a habit. The second time was after their usual Tuesday night dinner. Upon parting ways, he'd stepped forward and pecked her on the cheek. Then, at Cam's New Year's Eve party, he sought her out during the countdown and kissed her cheek, wishing her "Happy New Year, Bones," as he leaned in. She'd grinned and responded in kind, holding up her glass of champagne to clink against his beer. When he drove her home that night, he declined her invitation to come inside, saying it was late and he was exhausted. She surprised him then, when she leaned over and pressed her lips firmly against his cheek, offering a hasty but sincere "Thank you, Booth," before swiftly exiting the car and dashing up the path to her front door.
The habit continued throughout the New Year, naturally only at the end of the evening, and only after spending time together socially. They maintained their professional partnership with ease, and neither felt it necessary to discuss this new development at all. It felt natural and not enough of a change to warrant a serious conversation.
Months went by, cases kept materializing, they continued working hard and continued growing closer with each passing day. What they failed to notice was that their hands were reaching for one another more often, and not only in a show of comfort. Suddenly, when it was time to leave the bar on a Friday night, he took her hand when he met her at the door and they walked to the car together. After their mid-week dinner, she reached for his hand once she'd paid the bill (it had been her turn), and led him out of the diner. They failed to notice the kisses on the cheek lingering longer, or moving nearer to lips. They failed to notice that they no longer invited one another in after a night out - it just happened naturally. Booth would shut off the car and follow her upstairs, staying to have one more beer and share one more story before ending the night with yet another of those kisses on the cheek. Sometimes he would drive her back to his place with no mention of it. Then, Brennan would follow him up, and after sharing some late-night take-out, she'd tell him to stay put and called a cab, leaving her own lingering kiss on his cheek after the taxi honked outside.
More time went by, and they began falling asleep on each other's shoulders while watching a movie on his couch, or drinking one too many wines on hers. He would wake in the middle on the night, disoriented and still tired, choosing to remain under the blanket that had been draped over him at some point, rather than drive the short distance back to his own bed. She would wake only when it was morning, and instead of rushing back to her apartment to shower before work, she would rummage through his fridge and cupboards, starting breakfast before he emerged from his room, hair tousled by sleep and wearing nothing but well-worn pajama pants.
All of these things became routine somehow. Neither questioned it, neither felt it necessary to stop before it went too far. It took years to acknowledge to themselves how they felt, and months more to even begin any of these intimate gestures. Both were aware of just how slow the progression of this relationship had been; both understood that months added up to become years, yet neither made any move to rush along the changes.
---
It happened one thing at a time. First, they became partners; then, best friends. Of course eventually, they would become lovers. It happened without them really noticing. It happened so organically, that it wasn't until Angela caught them kissing (it was a quick kiss, a short meeting of lips before parting ways) outside the Founding Fathers one night that they realized what their relationship had become. She ran outside, unwilling to let them get away for another moment without explaining themselves, and when confronted, they honestly did have to think about how it had happened.
He suggested that it had begun the night that she'd fallen asleep on his shoulder a few months before. He had been brushing his fingers through her hair gently, lulling her into a relaxed sleep. She'd woken when he pressed a soft kiss against her forehead, and shifted her head up to look at him, her hand coming to rest upon his cheek when she leaned in and touched her lips to his.
She argued that it had begun long before that - last Christmas in fact, when he had pressed that kiss to her cheek before he left.
Angela laughed and shook her head, telling them both it had begun six years ago. Of course, she was met with vehement disagreement, but when she pointed out that every relationship, whether it be partners, friends, or lovers, begins with that first meeting, they had no argument in response.
Of course, being Angela, she pressed them for more - were they sleeping together? They were quick to deny this, and after flicking her eyes between the two for a moment, she chuckled and told them it was only a matter of time.
Despite the long-standing instinct to insist upon the platonic, Booth and Brennan found themselves unable to disagree.
---
It happened slowly.
It happened one thing at a time.
It happened eventually.
Because this was always inevitable, and because what they were moving towards was always going to be forever (or the foreseeable future), they were in no hurry.
The first time they woke up in the same bed, his body curled around hers, it was almost seven years after they met. They agreed that same morning that they should start paying more attention to the progression of their relationship. Because all their future inevitable moments were too important to miss.