Character A who thinks their vulnerability is a sign of weakness and will be used against them when(not if) they eventually get betrayed end up falling for Character B who thinks their vulnerability is a burden to those around them. So neither of them end up sharing their feelings to each other in a manner that's most frustrating.
A with their walls built high think "They deserve someone unbroken." while B with fear of draining the people they love think "They don’t need someone this messy." And it’s the most agonizing, slow-burn, mutual-pining, scream-into-the-void dynamic imaginable