Christianity observes that sex is meant for procreation, and any sexual act morally must therefore be ordered toward procreation. Contraception is directly opposed to this purpose, so it is a very grave sin regardless of the circumstances or relationship of the people involved. Note this is not necessarily the case for things which merely impede conception as a side-effect, such as some drugs or abstinence, provided the intent is not to prevent conception. (If the intent is to simply prevent conception, even abstinence can be sinful within marriage.)

We also don't see happiness in this life as a goal in the first place. The purpose of this life is to join in Christ's suffering for our sins; to know, love, and serve Him.

Finally, we believe that every human life is specially created by positive action of God Himself. Unlike animals, humans are not simply "random" biological by-products of sexual relations. This means that every child conceived is intended by God to exist, and therefore all action taken to prevent his life from coming into existence is necessarily action taken against God. If God does not want a married couple to have biological children of their own, they simply won't, despite taking no action of their own to prevent it.