By selfishness I mean his adamant refusal to stand aside. It leaves me with the impression that he believes no one but him should take the fight to McCaskill. Maybe selfishness isn’t quite the right word. Maybe it’s arrogance, or as you say, something as simple as confidence, misplaced or not.
Whatever it is, it’s not helpful in terms of making Harry Reid the minority leader, at least as far as I can tell.
Try pig-headed stupidity.