I bought a pet fancy mouse three months ago. He was six months old at the time, so fairly young and not well socialized. I've spent a lot of time with him, getting him used to being handled and around people. Just like my last mouse, he did nip me a little bit and I taught him that nipping was not alright. The behavior disappeared and now I have a pretty good relationship with him. He willingly climbs onto my hand and even when I let him out to play in his playpen he still loves to come and climb all over me.
Recently, he started nipping again, but the behavior is... different. I don't feel that it is aggressive. His nips do not hurt at all and are very light. Additionally, he is very slow and methodical with his teeth. I let him do it for a few seconds and what ended up happening was he just lightly held my finger in his mouth (no pain or breaking of the skin). I've been trying to discourage the behavior, but he seems intent on continuing.
I'm not really sure what to think about the behavior. Is it social? Aggressive? Something odd that only he does? Should I discourage it or simply let him continue?