If there's happiness to be found in Christianity, at least in this life, it's largely because we make an effort to be pleased by what God commands of us, what our final ends are as rational creatures. It's not because God is under some obligation to make us happy, and the fact that God got up on a cross and allowed Himself to be crucified does as good a job as anything can in showing how much priority "happiness" has.
On the flipside, Christianity is not a miserable religion. You are encouraged to be happy, very happy. Happiness is -good-. But here's the trick: *happiness* is a state. The state is fine, even commendable, in Christianity. *Doing some thing that makes you happy* is an act. Acts vary, and not every act is commendable. The difference is important.