Eid-ul-Fitr marks the end of the month of Ramadan. It is the day when the Muslims thank God for having given them the will, the strength and the endurance to observe fast and obey His commandment during the holy month of Ramadan. Eid-ul-Fitr affords an opportunity of spiritual reflection, after the month of Ramadan. A Muslim can now ponder over the strength (or weakness) of his will power; he can see, in the mirror of Ramadan, what were the strong (or weak) points of his character, because under the stress of fasting, the hidden qualities (or evils) of human character come to surface in such clear way which is, perhaps, not possible otherwise. Thus a man gets a chance of self-diagnosis of the traits of his character, which probably no one else may ever detect.
On Eid-ul-Fitr, the well-to-do Muslims are required to share a portion of what they have with their poor brethren. On the eve of Eid-ul-Fitr, a Muslim is obliged to give to the needy food-stuff at the rate of a prescribed weight, on behalf of himself and of every member of his family. It would certainly be pleasing to God if we did not forget these lessons after Eid-ul-Fitr.
Reference: Fast by Sayyid Sa’eed Akhtar Rizvi