“If I were hungry, I would not tell you, for the world and its fullness are mine. Do I eat the flesh of bulls or drink the blood of goats? Offer to God a sacrifice of thanksgiving, and perform your vows to the Most High, and call upon me in the day of trouble; I will deliver you, and you shall glorify me.” (Psalm 50:12-15 ESV)Does God "need" our praise? Of course not. God is self-sufficient; he needs nothing. In fact, it is not unreasonable to assume that if God needed someone to praise Him, he could come up with a better option than fallen, soiled, sinful old us. God has spotless glorious angels all around Him, for crying out loud. Besides... does God need someone to give Him a little boost to His self-image? Ridiculous! So why do we praise?
"The world rings with praise -- lovers praising their mistresses, readers their favorite poet, walkers praising the countryside, players praising their favorite game..." (C.S. Lewis, Reflections on the Psalms)
C.S. Lewis observed that whenever we enjoy something, that enjoyment automatically spills out into proselytizing. Did you just see a great movie? You want to tell everyone about it! How about a concert from your favorite band? Did you just see a rainbow? You're going to be talking about it! The telling, according to Lewis, is actually part of the enjoyment! The joy that you feel becomes more complete when you share it with someone else... especially if they enjoy it as much as you do. Maybe that's one of the reasons that God commands us to praise Him; until we do we cannot fully enjoy Him as He wants to be enjoyed.
For the record: I don't think the phrase "sacrifice of thanksgiving" was intended to have the meaning we give it these days. A "sacrifice" to us means something that is hard, undesirable, probably even painful. But if you look in the Old Testament, it was simply something that you were commanded by God to do... like tithing, or Communion. I don't think that struggling with the worship service makes it a "sacrifice of thanksgiving." To take the metaphor in context, you'd have to guess that it's a "sacrifice of thanksgiving" if we simply offer thanks to God in the way He has prescribed, whether the process of offering it was painful or pleasurable. Praise God to enjoy Him!