When doing a mitzvah it is essential to be happy. If we do not experience joy when we are serving Hashem our service will, G-d forbid, become a burden.
All the more so, when we are trying to help other Jews to fulfil their spiritual purpose do we have to do it in such a way that brings them joy. Joy endears the mitzvahs to them and this will bring them to do another mitzvah.
This is not referring to the joy of telling them jokes or giving them cookies. When we show them how a mitzvah opens the Gate to Heaven, that fulfilling a mitzvah produces a "time of favor," and since they listened to Hashem and did what He said, now He is going to listen to what they say…and then showing them how to open their hearts by speaking intimately to Hashem… all this brings a spiritual awareness and satisfaction, and this satisfaction brings a unique joy.
Mitzvahs are tools that we have been given that allow us to accomplish something unique in creation. In this entire, huge Universe only a tiny human being can choose to serve G-d.