when master bating I imagine...

Okay. So for some reason when master bating or whatever I can't think of my boyfriend and I having sex. For some reason I can only imagine him and some other girl. Or him having a threesome or something. I mean it works, I cum. But whyyy? If I try to imagine me and him it doesn't work. I love him very much and I know he loves me. So why am I having this issue?