Jesus has just washed the disciples feet a the Last Supper and starts his last discourse to them, teaching on true servant-hood. Jesus infers here that there is happiness that comes with humility.
This is one of the many counter-cultural teachings of Jesus. The world tells us it is better to be served than to serve. Jesus' model of washing the disciple's feet teaches the opposite. Happiness will be mine, says Jesus, if I serve others as though they are better than myself.