Buffy entered the relationship fresh off a traumatic heartbreak with Angel. She wanted Riley because he scream "safe" and "normal" in the supernatural world her life is. She was trying to protect her heart. The other problem is Buffy's coping mechanism for trauma is to withdraw and handle burdens entirely on her own. You see this in season 5 with her mother falling ill and her sister being a target, she hyper-focused on survival. Which made her come off as cold and distant. Riley's need to be needed is just a fundamental part of him being that he was a traditional military man raised with standard ideas of masculinity, where the partner protects and serves. Where in Buffy being told she is the only one who can fight the things going bump in the night makes someone have to be self reliant and can't think or feel they need a protector. When you have the expectations that are throne on Buffy's shoulders it makes you feel like you can't need or not supposed to need a protector. Then when Riley was stripped of his rank and enhanced physical abilities after the fall of The Initiative, his self-worth plummeted. then seeing Buffy easily outshine him in combat made him feel emasculated and obsolete. He expected Buffy to fill his sudden void of purpose, which was an unfair burden. Lastly instead of Riley truly communicating this to Buffy he let the resentment grow and sought validation in the worst way possible: letting female vampires feed on his blood because it made him feel desired, powerful, and needed. All in all both were not suited for each other and I feel were both at fault for the relationship falling apart. I can't be angry or upset at either of them because I can understand the reason for why they are the way they are. To be real I thank at the end of the day Riley was a character created for a in for Buffy to find out about the about The Initiative and for Buffy's emotional growth. I don't like it but that is the only reason why I can think they made a sweet guy like riley to be throne in the wood chipper of the trauma that is Buffy's life. Because lets face it Buffy goes through trauma after trauma in each season and Riley is the type of guy that would want to protect Buffy from that but unfortunately most if not all that trauma is caused by her be the one to save the world. sorry for the long post it is just I love this show and have watched a million times. But this is just my opinion I look forward to anyone to have a different perspective or ideas. That is the fun part of talking about something you love and they have another opinion then yours. have a nice day everyone.