Questioning the Fulfillment of Success?!

Is it true that after reaching a certain point in life, after achieving something you’ve been striving to accomplish for years, you often feel the most empty afterwards? People say we should always set new goals and keep working towards them, but does it really bring happiness? I mean, after becoming a millionaire, if you make another million, will it feel the same as when you made your first million, or does it hardly matter after a while?