[Drago]

I would agree that it probably has something to do (in a deeply psychological sense) with us living through the animals. The idea of passing on genes and wanting our offspring (such as our animals) to do the same. It also may be a parental instinct. Unless you're psychologically damaged, chances are you like or at least tolerate the idea of a small innocent creature needing your love and care. Here I think we see ourselves as the mothers/fathers/etc of the babies in a way.

It could also be curiosity. Humans are an extremely curious species, hence why we've advanced to the level we have. My other theory is perhaps competition/dominance. For some people, it's about having the most hamsters or being superior over their peers. I'm no psychologist, so these are just the thoughts of an amateur!