[@RumikoOhara]Tom Paris joined for the money.  Not sure why Baylana Torez joined, but her father wasn't one of the colonists in the demilitarized zone and her mother had nothing to do with it either.  Plus she was in Star Fleet at one time, so she could have gotten a job on a freighter in the Federation of Kligon Empire fairly easily.

Let's just say that he's emotional sometimes, and after feeling the pain of the Bajorans and human colonists that were hurt by the Cardassians, he wanted to help.  Joining the resistance seemed the most logical way to do that.