While there are many responses that could be made to this challenge, I will highlight just one: God certainly wants us to be “happy” – He wants our lives to be filled with joy. But do we sometimes confuse true “joy” (the kind of joy that comes from embracing Truth and Goodness) with a lesser joy that is nothing more than satisfying our carnal pleasure? Do we sometimes indulge ourselves in a physical and emotional high from the material world around us, when we could instead have the joy that is found in God? In other words, are we placing the “things” God has given us above God Himself?
As I said, there are many ways to explain why Lenten sacrifice is a spiritually enriching practice, but among these is the idea that separating ourselves from earthly pleasures allows us to focus more intently on God (where our joy will be complete). Giving up meat on Fridays, fasting, and choosing extra sacrifices to make throughout Lent, does not snub God by rejecting the gifts that He has given us, rather it puts God first, above the created things that we enjoy every other time of the year.