The entrance antiphon for the Third Sunday of Advent is
Rejoice in the Lord always; again I say, rejoice.
Indeed, the Lord is near.
The reason to rejoice is because we are getting close to the coming of the Lord. We ask in the Collect that we may attain the joys of so great a salvation and to celebrate them always with solemn worship and glad rejoicing. There is nothing sad about this waiting for the Lord to come. Isaiah will tell us that the desert and the parched land will exult; the steppe will rejoice and bloom. There will be abundant flowers and joyful song. "....they will see the glory of the Lord, the splendor of our God." God comes to save us...
I think that we need the comfort of this liturgy and even the pink candle signals joy and matches the vestments of the priest.
At the same time, we rejoice that Christ is coming soon, but we also keep preparing out hearts to receive Him. He loves to see us joyful. Let us remember our Advent resolutions and strive harder now that Advent is half over to please the Lord by our renewed effort. Come, Lord Jesus, and do not delay!