And then during battle Tony catches a grenade for Bucky and nearly gets himself killed (as he do), he's been blasted through a wall and he's unconscious and the suit's crushed in on his chest a little so he can't breathe, and Bucky's maybe a bit concerned—just a little bit—and uses the arm to just rip the whole front of Tony's suit open
And Tony breathes, and coughs, and blinks his eyes open and says, "So does this mean we can be friends now?" and Bucky startles the shit out of himself by actually laughing
And later Bucky's complaining to Steve, "God help me I think I actually like him, why Steve, WHY, HE IS THE MOST IRRITATING PERSON ON THE PLANET, WHY??" and Steve pats him on the shoulder sympathetically, "I know, Bucky, I know, he did pretty much the exact same thing to me"
Basically this is the only way Tony knows how to make friends and it's SUPER EFFECTIVE and nobody can figure out why