In 5x18, Eddie and Buck patch up the walls that Eddie had previously torn down. Eddie tells Buck "you're the guy who likes to fix things".
By 6x17, Eddie is at the hardware store with Bobby, specifically Bobby, and not Buck, discussing healing and being able to "fix" the broken parts of you (moving forward through grief for a loved one). And he goes on to meets Marisol in said store, and throughout season 7 attempts again to fix this perceived broken part of himself through her.
Significantly, Buck isn't there at the hardware store with Eddie. Even though Buck is always just randomly around Eddie in most of his scenes out of work, even though Buck has already worked on school projects with Chris this season. Buck is separated from this conversation; Eddie doesn't see him as a replacement for Shannon in this way. Eddie doesn't see the space that Buck holds in his heart as a place to be fixed or examined.
But by 7x10, we round back to Buck, there to help Eddie fix things. Marisol was not who Eddie needed and Eddie stumbled when faced with an opportunity to try and fix his own past mistakes, and Buck is there to do "what you always do".
Throughout season 7 Buck and Eddie simply CANNOT separate each other from their romantic lives and their journeys are so massively intertwined, often the only consistency within their messiness, and they keep accidentally colliding with each other at every corner. Wherever they turn, the other is there, ready to try for the other one.
