I think you're probably overanalyzing this entire thing. Your girlfriend is probably like me, the type who gets lazy easily and often does things based on "moods". I doubt your girlfriend hates the bass, but she wasn't playing it because she haven't been able to get into the right mood. I feel like I can understand where your girlfriend is coming from. I love piano a lot, but there are days when I just don't feel motivated to touch the piano at all.
And this doesn't have anything to do with how much I love piano, but it has more to do with my moods. When I'm in a bad mood, I just want to laze around and not do anything at all.