Disclaimer: I clearly don't own anything related to NCIS:LA. I barely even own a TV.
If asked, he'd say they became partners—real partners, not just employees assigned to one another—one day in her car when he'd fires off his explanation for his attachment to the Beretta 92FS and then holds it out to her butt first, a silent statement that I trust you to have my back. She doesn't touch it; he hears her own silent statement that I need to know you trust me, not what the gun feels like in my hand. It feels like a hard-won prize, that decision on both their parts to do what is contrary to their own natures—he to trust someone with that intensely personal attachment to his gun (and, by extension, his safety), and she to deny her own curiosity in deference to another person's comfort. It's then that he begins to be not just willing to let her fire it, but almost eager for it.
When, a few weeks later at the practice range, he cheerfully challenges her to a contest—with the other person's gun (I'll lay mine here, you lay yours here—on three, grab, load, and fire as fast as you can til you empty the clip. Highest score wins), she doesn't say no. She still beats him—he shouldn't have expected any different—but when their targets roll in, each sporting tight center mass groupings, he realizes they've taken a leap into the deep end of trust and vulnerability.
(He thinks there's an added bonus there in the knowledge that they both now know that their partner can pick up an unfamiliar gun at any moment's notice and not be thrown off their game.)
He thinks they became friends on a cement floor outside a room that had just been disintegrated by explosives after they came flying out of it, hand in hand. If asked, he'd tell you that that's the moment they became more than just functional partners and moved into something personal. Before that day, they were only slowly moving toward completely trusting each other in the field. There had always been something between them that snapped and sizzled with a charge they didn't really understand, but there on the hard cement when she looks down at him and grins and they realize they're still alive, something moves them beyond the line of colleagues into a deeper realm of intensity.
(Maybe it was his comment about having just peed himself. Maybe it was the quick sweep of his hand to brush her hair back from her face. Maybe it was the hovering of hers as she held back from touching his cheek in the rush of adrenaline that came from exhaustion and explosions and emotion.)
He thinks he started loving her then, in a way. Not the same love that he feels now—this all consuming thing that requires her to be the first thing that he sees when he wakes and the last thing he hears before he sleeps, that makes the thought of her with anyone else feel like death, that makes him see a little house on the beach and a little room painted pink and makes the possibilities of the future look more like the gleam of gold, like red and green Christmases, and like baby blue and pink instead of the vague grey that once was his expectation for the future. Not that love, yet, but the first flickers of it, a sort of affection that wasn't temporal or superficial either.
He loved her then, he thinks, like a friend. Like someone he needed to know was present and safe somewhere nearby. Like the first person in a very long time that would make the real laugh come out. Like someone he might care to see after the end of the work day and who might fill some of the empty spaces in his life.
He decides that she's his best friend, ironically, while Ray is there. He's standing beside the man that he's always called his best friend, the one he went to hell and back with more than once, when he realizes that the woman on his other side is really his best friend now.
Ray is still, certainly, his oldest friend. He is still one of his closest friends. But the years have changed them, and while the bond they have is going to be there til death do us part, they're from different worlds now, and they don't understand each other quite the same way they used to. The two men for whom home was synonymous with pain and turmoil growing up had made their own space to belong with each other—they'd sworn as blood brothers to have each other's backs and to always have a place for the other wherever they were. They'd faced death and poverty and violence and fear together, and survived it. They're still brothers, Deeks knows, but they've made real homes now in different places, grown up homes for grown up men. Ray with Jennifer and this tiny new being that they're bringing into the world, and Deeks with NCIS and Kensi and Hetty and Eric and Nell and Callen and even Sam.
Deeks is pretty sure Ray knew before he did that Kensi had usurped the "best friend" position. And though Deeks isn't ready to agree with Ray's speculations on their thing, he can't deny how important she's become.
If he tried to pinpoint a moment when he realized that what he felt for her went beyond physical lust (because, really, everyone knows he's felt that since he first laid eyes on her), beyond partnership, beyond best-friendship, he'd say it was one night sitting outside a bar with a tattooed and dolled up Kensi playing bait for the bad guy a short distance away. He's always gotten a queer feeling in his gut when Kensi has to dress up to seduce a bad guy. (When he's honest with himself, he thinks he'd probably still get it if she was dressing up voluntarily to attract a normal guy.) That night the feeling is stronger, and eventually he recognizes it as a new feeling of possessiveness that Kensi definitely would not approve of.
For a minute, when Kensi's "please. I am so your type" came across the comms, he thinks maybe he's fallen asleep and dream-Kensi is talking.
But it's very-much-awake Deeks that feels the thrill of those words and realizes for the first time how very right she is. Kensi is so his type. Except that his type has always been shorter and blonder and gigglier. But, suddenly blonde and giggly sounds annoying and boring, and when he tries to define his type he just sees Kensi. He likes her sense of humor—or maybe the lack thereof (he likes to be the funny one, after all), he likes her serious side, her sense of justice, her fierce determination, and her loyalty. He likes working with her and fighting with her and figuring her out. He likes that tentative look she gets when she's feeling insecure, but mostly he likes being able to wipe it off her face by reminding her how amazing she is.
And all that on top of her other obvious assets.
(More than that, he realizes, now he really wants to be her type.)
If asked to pinpoint a moment, Deeks would say he knew he loved her the first morning he woke up in bed alone after they returned home from being Justin and Melissa. He reaches for her instinctively and the other side of the bed is cold and the pillow doesn't smell like her, and something in him revolts with a force that surprises him.
(He figures out pretty quickly that it's his heart.)
Making breakfast alone feels wrong, and he fights the urge to make pancakes and then call and taunt her with their existence until she comes over to steal some. He realizes swiftly that everything now feels wrong without her there beside him.
The unsettled feeling doesn't go away again until he convinces her to pick him up for work and she pulls up in the driveway, already taunting him for one thing or another, but with his favorite coffee in the cup holder. He slides in the passenger seat and something in him (darn heart again) settles and clicks into place, and he knows, staring at her profile as she drives, that he hadn't really ever understood what love was until that moment.
(Almost every day for a year and a half after that, he secretly discovers that he still doesn't understand what love really is, because every day it means a little bit more, gives a little bit more, requires a little bit more of him.)
He's pretty sure the Cronut is the turning point in all the aftermath. Four months in and he still can hardly function. Four months, and she hasn't quit trying, even though something in him has been unable to let her in, until finally her presence chases away the edges of the darkness enough that he can finally start to see beyond it. He almost doesn't open the door. He hasn't, actually, opened his door for any reason in days—which probably explained how he's missed her gift up until then. In the end, the prospect of having to look out the peephole and see the hurt in her eyes if he doesn't open the door is worse than the prospect of having to face something of the outside world. Even in the depths of this hole that he can't find his way out of, he'd still rather take the pain himself than see her hurting.
But then he pulls a lumpy stale pastry out of a greasy paper bag, and everything else disappears. He's always kind of wondered if people actually listened to what he said, or if they just smiled and nodded in the right places until he stopped. Then he pulls a Cronut out of a takeout bag, and it hits him like a ton of bricks. She was listening. Not just listening, but remembering. And remembering the silly little things. She cared enough to fly a little pastry two thousand eight hundred miles across the country to try and cheer him up. And he hasn't even looked out long enough to see it. It's what he had needs to force him back into life again—a confirmation that if he doesn't need to live for himself, he still needs to live for her, for them. For months, his warped brain has been telling him that this was the end of the story, that there isn't going to be any more to tell. The little round donut hybrid seems to whisper at him that maybe he's been wrong. Maybe there's still more story to tell. More than that, maybe it's not the end of the story, but only the beginning. And maybe the story really is a love story. So, for the first time he feels like maybe he can find the light again. Because they matter, like nothing else has ever mattered to him before. If she wants him, then he'll be there.