I believe that Buffy needs passion and emotion to be able to fight the way she does, and she gets that from many places, such as when her mom died, Dawn was captured, and Spike gave her his "You're the one" speech". Buffy entered into such a violent relationship with Spike so she could feel something real after she had been pulled out of heaven. He made her feel passion that she had never felt before, feelings that she liked in the moment but scared her and made her ashamed when they were not together. Spike interprets her fighting on passion and emotion as the need to be inlobe with men that hurt her and having that as the fuel when she fights. Yes, Buffy was very determined and powerful when Angel became and Angellus and repeateldy hurt her but there are many other examples where men are not at play and she fights extraordinarly. I also do not believe that Buffy is attracted to violent me as both Angel (until he Lost his soul) and Riley were both pretty easy going guys. She went for Spike because A) he pursued her and had evident, true feelings for her and B) he made her feel something she had never felt and that she wanted to continue to keep feeling. Buffy and Spike's relationship was so different from her past dating history for many reasons, and the biggest one is arguably (other than the whole she was pulled from a heaven dimension) because she was plus mature than she had been with Angel and Riley. She had grown up and this was her first real adult relationship.