If this were Jesus about to serve us, we would be shocked. We can understand why Peter protested when Jesus stooped down to wash his feet. We too would resist if Jesus were our waiter in a restaurant, a practical example of how most of us experience being served today.
But this is not a painting of Jesus. It is a priest. The priest is called to imitate Jesus in serving. In the hierarchy of deacons, priests, bishops and popes, the servants serve the servants, making authority and servitude to grow together.
Just as there is no greater master than Jesus, there is also no greater servant than Jesus. He is the prototype of both the master and the servant.
Thus, Jesus invests the priest with His authority in dispensing the sacraments, and in preaching, while charging the priest with servitude in the giving of himself, that more truly marks the priest, the man of Christ.