Reason for leaving job leaving excuses

I have decided I’d like to leave my current position. It’s just not a good fit for me and I’m not very happy here. I’ve decided to try my luck at temp agencies but I don’t know what to say when I’m asked why I would want to forfeit such a “good” position.

In all honesty, I’d like to return to school and I wouldn’t mind getting a perm position but at the same time I don’t want to feel trapped. It’s just something about a temp position that would make me feel a little less locked-in.