The Chofetz Chaim writes that because we are so involved in worldly matters, we lose our sensitivity to the great amount of joy we can potentially experience when performing a mitzvah (good deed). He offers the analogy of a man who was granted an audience with a powerful ruler. Imagine that the ruler is greatly impressed with the man, and has the conversation recorded in his personal diary. What a thrill! Upon returning home, the man's face would glow with elation as he retells his experience to all his friends and neighbors. Even if he'd previously been worried over personal problems, he'd quickly forget them! Over the next years, whenever he'd meet others at some gathering, his successful meeting with the ruler would invariably be the topic of conversation.
Says the Chofetz Chaim: If this is the joy of someone who found favor with a mortal (who will eventually die and whose glory is short-lived), all the more so should we feel joy when we doing something which finds favor with the eternal Creator of the universe. Even afterward, when recalling the good deed, we will feel a glow of pleasure. In fact, the Torah (Deut. 28:47) stresses that we should feel more joy in serving the Almighty than from all other pleasures that exist.
(Shaim Olam 2:11; cited in Rabbi Pliskin's "Gateway to Happiness," pp.91-2)