He is obviously checked out mentally compared to where he was earlier in the season when we were still competing. Losing 10 games in a row, being completely out of contention, having heavy legs from playing heavy minutes because of injuries, being down by 20 points for most of the minutes you're on the floor, having opposing defenses surround and collapse on him in the paint because they don't fear our 3 point shooters, rarely getting the inlet pass because of ball dominant guards... these things take a toll on motivation and focus.
Not everyone can be like Sexton mentally, and even Sexton's focus has dropped off at times.
I don't blame him even a little bit. It's called being human. If we were in contention and if we had better pieces with him on the floor I think you'd see a different player out there. If we're bored and hopeless as fans imagine what it's like as a high minutes player banging in the paint every night.