The theme of Advent (from Latin that means "coming") is the incipient presence of Jesus in our lives - his presence with us today, his presence at his arrival in Bethlehem, and his second coming in the final Advent.
This third Advent Sunday focuses on John the Baptist and Joy. This Sunday is also called Gaudete Sunday (from the Latin meaning "joy") and the pink Advent candle is lit. While happiness is ephemeral, joy is the permanent feeling of knowing that God loves you and nothing can take away that joy.
This state of joy is related to the theological concept of Assurance - all Christians who have faith will feel assurance that we know we are loved by God and our sins are forgiven.
I felt my heart strangely warmed. I felt I did trust in Christ, Christ alone for salvation, and an assurance was given me that He had taken my sin, even mine. - from Wesley's Journal
Be joyful and know you are loved!