ולא יכל יוסף להתאפק לכל הנצבים עליו… (ויגש מה:א)

And until now, Yosef was able to control himself?  However, Yosef used strict words to belittle his brothers and break their hearts to cause them to do teshuvah – repentance for selling him.  Until they were subjected entirely to Yosef, and they spoke with arrogance, Yosef reciprocated and acted as a stranger to them.  But the minute Yehuda said, “I’m ready to be your servant,” he showed interest in Yosef, and Yosef couldn’t restrain himself anymore.