Pleasure by itself does not bring happiness. We can experience pleasure (e.g. eating, sleeping, sex) without an investment of psychic energy. Enjoyment on the other hand, happens only as a result of an unusual amount of attention. Pleasure is fleeting and, unlike enjoyment, does not bring complexity (growth) to the self. If one only invests energy in new directions solely for extrinsic rewards, one may end up no longer enjoying life, and pleasures become the only source of positive experience. Without enjoyment life can be endured and can even be pleasant. But it can be so only precariously, depending on luck and the cooperation of the external environment.
From this article by Mihaly Csiksczentmihalyi, the author of the wonderful book on Flow. This article presents a good summary of the main ideas in his book.