Tony didn't know when it happened, he just knew it did.

Maybe it was when he'd stumbled into the kitchen after spending 4 sleepless nights in the lab, trying, and failing, to make himself coffee until a warm hand wrapped his fingers around a steaming mug filled with it, and he'd looked up to see Steve smiling with a look of understanding on his face.

Maybe it was that time after a particularly bad mission, and no matter how many times Tony ran the calculations in his head, he could never eradicate the chances of civilian deaths, and some of them were fucking kids, and just as he was about to slam the alcohol bottle he'd drained onto his head, Steve came in to stop him, took the bottle out of his hand, and hugged him so hard, the tears came out and wouldn't stop.

Or maybe while the team had assembled early in the living room, preparing to leave for a mission briefing, and he looked over at Steve watching the sunrise by the window, and the sun hit him just right and Tony couldn't get himself to breathe.

Either way, he had fallen for Steve Rogers, and he wasn't sure what to do with all the feelings that just didn't want to leave him alone. He knew it was different, that Steve was different, and he was so terrified of acting on what he felt because he didn't want to hurt him. Ever.

But then, that decision was made for him, and he didn't need to be so confused about it anymore. It was one of those nights where neither of them could sleep and ended up in the living room, keeping each other company and talking about nothing, and when there was a lull in the conversation, Steve just looked at him, leaned forward, and pressed his lips against Tony's, and Tony's usually noisy, non-stop-working mind went dead silent.

"I'm sorry." Steve said, when he pulled back, and he looked so ready for rejection, and Tony couldn't help but smile instead, taking Steve's hand and tangling their fingers together, and replied, "For what?"

The blonde looked surprised, blush worming its way onto his face, and Tony found it so extremely endearing and adorable, and that's when he thought: 'God, I love you.'

"You do?" and Tony had to take a few seconds to realize that he'd said that out loud, and he scrambled to cover it up, because he didn't want to scare Steve away, not after just discovering this.

"I-I...well, yes, but you don't-you don't have to feel that way back, or anything, you know, that's just me, don't feel, you know, obligated or anything, because we'd only just had our first kiss there and-"

Steve cut him off with a soft chuckle, shook his head, smiled that smile that made him look like a kid on Christmas morning, lighting up his whole face and giving Tony that feeling of just wanting to give anything to keep that smile there all the time, saying:

"I love you, too."

And that was it. The deal was sealed. Tony's flawed, weak, heart didn't belong to him anymore, because it was Steve's, and maybe it had been for quite a while now, he just never really realized it.

Sure, a lot of people could call Tony Stark a selfish son of a bitch, but as long as he got to keep Steve, he didn't need anything else.

He certainly didn't need his heart back, either.