See, the thing is, I don't think it was a conscious decision at all. I don't think Tobias rationally thought, "well, my life sucks, I want to live as a hawk, but on the other hand, my life is getting better and maybe I shouldn't, oh what the heck I'll do it." I think it was a sort of mixture between an accident and a decision. I definitely don't buy that it was completely an accident (for reasons I've stated a long time ago in this very thread, incidentally). But I don't think it was completely a decision, either. Tobias seemed to truly believe that he still wanted to be human, right up until book #13.
What most likely happened was that some subconscious urge to abandon his human life and stay a hawk made him hesitate, even when he consciously knew he should be trying to escape. Does anybody know that feeling? It's like forgetting to get gas on the day you're supposed to go to the dentist. Consciously, you aren't actually thinking "well, how can I give myself an excuse to be trapped away from something I don't like?" yes that sentence was intended to have a double meaning But at the same time you probably wouldn't have made that mistake if you didn't subconsciously know that you would benefit from it.