Schmoopiest headcanon alive:
Tony, the ridiculous asshole he is was one day so giddy over something he jumped up on Buckys back whooping and to his delight his boy didn’t even flinch or stumble, strong super soldier that he is. So later, tony did it again. And again. Bucky all fake exasperated tolerated it, but one day, he just wrapped his elbows under tonys thighs and held on so he couldn’t get down until he started whining. Little did Bucky know. Tony loved it. And now, its a thing. Just imagine in your head. Tony fuckin piggy back on Bucky as he goes through his mundane everyday tasks ok. That’s what this is about. Clingy koala tony. Fight me.