Steve and Bucky were together before the serum, and afterwards, they didn't have a ton of private time.
Fast forward to the future, Bucky is back and they're together again, except romance got quite a bit different. Like, Bucky is totally used to bending down to kiss Steve, except now he's taller, and he can't exactly dip Steve without his arm practically giving out. And it used to make sense to have Steve sit on Bucky's lap, but now Steve's legs have nowhere to go, so it becomes more of an awkward tangle. And Steve used to be able to jump into Bucky's arms, and Bucky may be stronger now, but momentum is still a thing, so they crash.
Basically, they finally have time to be together and affectionate, and it takes more than a little finagling and relearning to do so.
Steve/Bucky, Steve is still used to being a little guy
Fast forward to the future, Bucky is back and they're together again, except romance got quite a bit different. Like, Bucky is totally used to bending down to kiss Steve, except now he's taller, and he can't exactly dip Steve without his arm practically giving out. And it used to make sense to have Steve sit on Bucky's lap, but now Steve's legs have nowhere to go, so it becomes more of an awkward tangle. And Steve used to be able to jump into Bucky's arms, and Bucky may be stronger now, but momentum is still a thing, so they crash.
Basically, they finally have time to be together and affectionate, and it takes more than a little finagling and relearning to do so.