Right now, Steve is lying in bed. Tony's bed, to be exact. The space next to him, though, is unoccupied. Steve is used to this by now. He knows Tony has never been one to keep a regular sleep schedule.
Tony is currently in the shower, not caring that it's only a little past three in the morning. Tony always tries to be quiet, and Steve always pretends to stay asleep as he gets out of bed, peeking through half-closed lids as Tony moves about the room.
Today, though, Tony is singing quietly to himself, muffled by the sound of water and the almost-closed bathroom door. Steve hardly notices it at first, and when he does, he can't help but smile. Though Tony can be sarcastic, impatient, and flippant, Steve knows there are so many other parts to him. If he were asked if one word could describe Tony Stark, Steve would say "guarded;" though Tony might say "genius," or "billionaire." At that thought, Steve gives a small laugh; the difference in would-be-answers is the point.
In the several months they'd known each other, Steve had learned quite a bit about Tony. While their relationship had started off rocky, it quickly improved after Steve and the rest of the Avengers moved in with Tony. They began to get to know each other, both being plagued by frequent bouts of insomnia. When it started, the two would sit for hours at a time, drinking coffee while Tony showed Steve different things he had missed over the last several decades. As they grew more comfortable with each other, their conversations grew deeper and more personal.
One night, he and Tony were sitting on a couch in Tony's workshop, while Steve told him about some of his childhood. As he trailed off at the end of a story he noticed Tony, who was barely awake at that point, start to lean over on to him. Steve smiled, and effortlessly scooped him up, to carry him to bed. When they reached Tony's room, he set him down gently on the bed, and left for his own room. This began to happen more frequently over the following weeks.
Steve would say that their relationship started in that time, particularly one night a month or so in to their routine. As Steve leaned down to lay Tony on his bed, Tony held onto his arm, mumbling "stay," so quietly that Steve could have missed it. Unsure of himself, he sat down with Tony, who tugged him over with Steve's arms still wrapped around him.
Blinking, Steve hears the water turn off, though Tony is still singing gently to himself. Tony slips into the room quietly, not knowing Steve is awake. He shuffles into a grey tank and black sweats, and heads for the door, probably to go down to the workshop.
"Stay," Steve whispers, startling Tony just a little. Tony turns slowly, smiles softly, and climbs back into bed. Steve runs his hand through Tony's still-dripping hair, and then wraps his arms around him, with Tony's head on his chest.