Listen, I don't want him to cheat on his girlfriend and I also don't want to be his mistress or whatever. These are things I want to avoid (but it may be too late).

I just know a guy who has showered me with attention and affection, with no apparent motive for doing so, and I'm falling for him. He has a girlfriend. It sucks, and I cannot explain why he expends all this energy on me. That's all.