Why doesn’t success bring happiness? What does?

Ans :
It’s because people base their ideas of success on outdated models or patterns of what it means to be successful. For example, someone might have the idea that in order to be successful he or she must have certain kind of house, a certain kind of car, and a certain kind of spouse. They’re using those things as evidence for whether they are a success, but they’re leaving themselves out of the picture. They don’t everactually step into that picture and ask, “Do I really want to live this type of life?'” So I recommend that
people not merely visualize what it would be like to actually live the type of life they are trying so hard to achieve, but to step into that life and mentally experience several weeks or months. When people do thisexercise, they may find there are things they want to change, and I recommend they make those changes.After all, since they’re working so hard for this future, it ought to be one they’ll enjoy when it arrives.

Eknath Easwaran has another good explanation on the same topic

The external world, so fascinating, so infinite in its variety, has us firmly in hand and thoroughly mesmerized. Lasting happiness is almost ours – over there, just ahead of us, right around the next corner. When we round that corner and find it has eluded us, something in us says, “Keep running! It’s just around the next corner.” Finally, our life becomes a continual pilgrimage around corners. Such is human credulity that even after rounding a thousand corners, we still say, “The thousand and first, that is the corner.”

If we believe that happiness arises only when some external condition is fulfilled, we consign ourselves to a perpetual state of discontent. For even when our expectations are fulfilled, sooner or later the little voice inside starts again, “More! More!” It is this habit, this almost mechanical fixation of the mind, that keeps us forever chasing rainbows, until at last we begin to suspect that the kingdom of heaven is within.