"You yourselves bear me witness, that I said, 'I am not the Christ, but I have been sent before him.' The one who has the bride is the bridegroom. The friend of the bridegroom, who stands and hears him, rejoices greatly at the bridegroom's voice. Therefore this joy of mine is now complete. He must increase, I must decrease." (John 3:28-30, ESV)
It takes but a moment in reading the Apostle John's gospel account to get a sense of what motivated John the Baptist - to get a sense of this man's heart. In John 3 we read of John the Baptist making his mission crystal clear. He understands his life's calling to be one that testifies to Jesus. He was born to show the world who Jesus is and why he has come. John the Baptist actually says that this mission is where he finds his joy. He even makes it more explicit than that. In John 3:29 we read this coming from the mouth of John the Baptist, "Therefore this joy of mine is now complete." Do you see what he has just said? John has said that the pinnacle, the apex, the culmination of his joy is found in the person of Jesus. And not only in the person of Jesus but in John's association with Jesus - in John's pointing the world to Jesus. John was born to show the world who Jesus is - to help bring the world to Jesus.
The amazing thing is that if you love Jesus this is what you were born for too. If you call yourself a disciple of Jesus, you can be sure that you were born to follow in the footsteps of your great hero of the faith, John the Baptist. When that truth becomes clear and aligns inside of you then John's statement in verse 30 makes perfect sense, "He must increase, I must decrease." This is where your joy is found because this is what you were created for.