Bucky loves Tony, but thinks Tony and Natasha are a couple. Tony loves Bucky, but thinks his feelings aren't reciprocated. Natasha knows all, and thinks it's about time these two idiots get together already.
Alternately: The one where Bucky is jealous, Tony is clueless, and Natasha is 100% done (and decides to take matters into her own hands).
Bucky is enjoying his new, post-Winter Soldier life at Avengers Tower, until he discovers that the constant tension between Steve and Tony was caused by a recent (and mysterious) breakup. Determined to make his friends happy, Bucky gives himself a new mission: figure out what went wrong, and get these two idiots in love back together again.
Bucky felt like there were a lot of things about the world and his life that reminded him on a daily basis of how much he’d changed, but oddly enough the continual sticking point for him wasn’t his missing arm, or the lost time, the anxiety attacks, nightmares, or the way his memory was like a sieve; it was Steve and Tony. Specifically the fact that he couldn’t figure out what the hell was going on between the two of them.