The thought of reward in Olam Haba generally does not make much of an impact on our daily decision making. First it is too far off but second and more importantly we can't relate to what Olam Haba is all about. Maybe if we saw a brochure with nice glossy photos things would be different. But we haven't so it isn't. Nevertheless the thought of Olam Haba provides at least a small incentive to perform mitzvos so it is important to frame a mature picture in our mind. We cannot go our whole lives with our childish images of having all the toys and treats we can imagine, says Rav Shimshon Pincus (Sichos Purim - Mishenichnas Adar).
Olam Haba is eternal which means that we must be entertained forever. Quite a feat since we easily get bored of even the greatest of pleasure as the thrill wears off. Justified or not our attitude is "out with the old and in with the new." Eternity is a very long time to remain excited. So how is this achieved? Rav Shimshon answers with the Vilna Gaon who explains the Pasuk, "Oz Yimalei Sichok Penu." Sichok or laughter, like happiness, is short lived. If someone were to tell you that you won the lottery it would be the greatest moment of your life. A truly carefree moment of pure enjoyment. This is Sichok. The next day you are very happy but in a more calm and calculated fashion.
Not so in Olam Haba. Every moment will be Sichok; pure unadulterated ecstasy. Before you have a chance to come off your high, you will be given another enjoyment that will evoke an additional outbreak of Sichok. It will be a constant state of ecstasy, new thrills coming at you moment after moment for eternity. That is what the Pasuk means "Oz Yimalei Sichok Penu"; only in Olam Haba will our Sichok be complete. In this world it can't last long, but in the next world the Sichok will be whole, with every single moment bringing us new thrills.
This he says is the inyan of Purim. We were signed and sealed by Hashem for destruction. That verdict was not repealed, rather Hashem created us anew. Purim is day to celebrate being newly created. It is the day we won the lottery. We have no care in the world. It is a day of pure uninhibited ecstasy. No special obligation to Hashem. The only obligation is to realize what we were given and breakout in endless joy and happiness.