Its nothing that concerns Angel or Buffy's relationship. That is was in Season 4. Well in Season 5 when Buffy discovered about her mother's mysterious illness. Buffy was too concerned for her mother. In that whole series Buffy didn't pay any attention to Riley in the season. Riley feels that Buffy doesn't show him her love. So Buffy knew what drive Riley the most that Buffy was becoming to strong that he can't touch because her strenghtn.
Why does Riley leave?
After Riley losses his super powers from the militaries drugs and Buffy has super powers he feels less than a man. He's always getting hurt and always asks Buffy not to let up on him when they're sparing. As time goes on and Buffy is taking care of her sick mother and taking care of Dawn she forgets to tell Riley and he finds out through the scoobies what's going on and he feels Buffy's not letting him in. So he can start feeling like a man he takes risks that are not healthy for a person. Spike shows Buffy what Riley's doing and he tells her its because she's not letting him be part of her life.
When does Riley leave?
Riley finds out that some of his old military buddies are going on a job in South America to take down a vampire nest. He gives Buffy an ultimatum that if she wants him to stay she has till midnight to met him before he takes off. She shows up late and Riley's up in the air but doesn't see or hear her and goes off!!!
What did Buffy do to Riley that he left?
Nothing but take care of her life. What Riley did was his own fault.
Was it Angel or Buffy or Both?
Part of it could have been Angel because Riley wasn't feeling man enough and thought that Buffy loved the dead. Buffy and Angel not way because I think that she loved him especially after Zanders speech. Both no because it wasn't about Buffy being with Angel because it didn't happen.
Was it because Buffy had him quit the military scientist? No because Riley quit them on his own. Buffy just helped him get better by turning him in to the military.
Complicated question to a complex story - I mean Riley Finn was basically like Jacob in Twighlight saga only he was completely used as a baby sitter and she did not respect him enough to consider his feelings. Nah - I watched all the shows and after she stood him up (when he left) I guess I did not like her much... I mean she had already started her affair with Spike by then.. nah if she got there in time to see Riley, she would have used him again. He did give blood to vamps but I guess he wanted to become one. He wanted to be what she wanted... that's how much he loved her. He was just a guy in a really weird world and to be honest, he could have done much better than buffy... I was hoping he'd come back and some how save all of them but then reject her ever so nicely... some-thing like - sorry buffy but I don't feel the same way I did when we were children. I grew up and moved on. I have found myself in a much happier place now - take care of yourself and all the dead things.
By the time Riley left, our house started calling the show "buffy the vampire player"
Sadly it was Dawn who mentioned him most after that. Good on ya Dawn... I missed him too...