In "The Bet" by Anton P. Chekhov, what motivates the banker to participate in the bet

The banker believes that capital punishment or death penalty is more humane than life imprisonment. However, a young lawyer disagreed his view, saying that he would rather live a longer life in prison than suffer death.

Because the confrontation happened in the party hosted by the banker; where important guests bore witness, the banker wagered against the belief of the young lawyer to save face and insist that his view is the right one.