I really don't think it's an attitude issue or a deal where Lewis is in Prohm's dog house. When players get in a coach's dog house, generally the writing is on the wall, see Cam Lard last year, BDJ toward the end of Fred's final disaster year. If it was something like that, I don't think Lewis would've come back for year 3. If it was Lewis thinking he's too good to practice or better than the guys playing over him, he would have left of his own volition or Prohm would've convinced him too.
My guess is actually the opposite; he has a GOOD attitude. He is a GOOD teammate.
Maybe he doesn't have a competitive edge to him, maybe he has all the skill but lacks the confidence or aggression. Maybe he loves the team and the school and just wants to help in whatever way he can, whether that is by encouraging those who are earning PT or by challenging them to get better in practice every day. Maybe he realizes that his mentality is as more of a coach than as a player, and he's more of an asset to the team in that regard than by his play on the court.
One thing I do know and have realized is the lack of physical development. Remember when he came in a lot of folks on here thought maybe he could fill a Hogue/Ejim type roll down the road for us. He has the height, 6'5'', 6'6'', and seems to have a frame to add some muscle. My eye test tells me that he is still pretty skinny even after 2+ years at a P6 school with P6 weight/nutrition facilities. Maybe that ties back in to the work ethic theory. At the end of the day, who really knows? I would like to have the answers, but I doubt we ever get them.