I don't know if Stevens instills toughness so much as he channels it. The toughness itself is a credit to the players. Ainge specifically seeks those guys out during the draft evaluation process. If he has reservations about the competitiveness of a player, he'll back off. Identifying players with the right temperament is probably a bigger part of the job than managing it. Rozier, Tatum, Horford, Smart, and Brown came out of the womb with an edge. That's not Stevens.
But "fire" and "toughness" and "competitiveness" can turn to selfishness or anxiety real quick, and to the extent that Stevens has been able to nurture those attributes into a winning formula is a credit to him.