Jesus had sent His disciples out to spread good news. They were to go to a town and discover first whether the Holy Spirit had been busy there. They would go up to a house and wish the people peace. If the people had peace, then the disciples would know that the Spirit was at work there. Stay in that house, Jesus told them, and share what you've heard from me. And also, by the way, drive out demons and cure sicknesses.
Well, they did it. And they drove out devils and cured sick people. So when they returned, they were pretty happy. Jesus could see that. Jesus also knew that a happiness that depended too exclusively on success is a temporary happiness. So He warned them about depending on success, on doing well, for their happiness.
Instead, base your happiness and contentment on this ultimate truth about yourself: Your name is written down among the saints in heaven. The next time you try to be loving to someone and it doesn't work, you won't have to be dejected. Disappointed, but not dejected - because disappointment is as temporary as success, and what keeps you happy is consolation. It is not temporary, though it tends to have fevers and chllls from time to time.
Our consolation is that we have been baptized in Jesus Christ's death and resurrection. That is a permanent cleansing, praise God!