The origins of the modern seder are in the discussions and commentaries by Jewish sages, particularly the mystics. They felt that in particular it was crucial that each person be actively involved in the retelling of the Passover story. Asking questions -- being in a questioning state -- was seen as crucial to the "mood" of the seder. As a result, we do a lot of unusual things at the seder, in order to invite wonderment and interest. For example, normally when one does a ritual handwashing, a bracha (prayer) is said. However, on Passover there is an additional earlier handwashing which is done without prayer at all - why? One reason, the Rabbis explain, is to make people ask the question why.
According to the Mishna, you are only obligated to recite the Ma Nishtana if the child does not spontaneously ask his or her own questions about the seder. A popular commentary in the Haggada states, "The Gemara tells of a story of young Abaye at a Passover seder. Just before they are about the recite the Ma Nishtana, the servants rush up to the table and remove the seder plate. Abaye asks, 'Rabba, why did they remove the seder plate?' To which Rabba answered, 'You have fulfilled the obligation of asking questions; we no longer need to recite the Ma Nishtana.'"