If he loves us, why does he keep re offending?

My partner is in prison and misses me and the kids loads, but this is his third sentence since 2008. I'm having a hard time even believing he loves and misses us because why does he keep offending? I ask him when I visit him but he can't give me any straight answers. He changes the subject every time and spins it around on me asking if I'm being faithful and so on.
Our son is confused and I don't think its fair on him me having to take him prison visiting all the time. I am not having his Dad's selfish stupidity have an impact on our son. I am losing faith and interest in my partner and unsure whether to leave him to it. I don't like visiting him anymore and it is becoming a chore. I know this is a decision only I can make but I want a proper family and me and son prison visiting all the time is not what I call family life. If he loves and misses us as much as he says he does then he wouldn't keep re offending. His poor choices are making me depressed and I don't want my son feeling my negativity. It's just not right. Thanks for listening, A partner of a prisoner.