A student only gets to learn this charm after one has passed the O.W.Ls (Ordinary Wizarding Levels) as to produce a Patronus, one requires a lot of strenght and endurances. The whole purpose of conjuring a Patronus, is to drive away Dementors amd to protect oneself from them. But one must also be able to sustain the charm, as it requires a lot of energy.
The Patronus is a kind of anti-Dementor guardian that acts as a shield between the person conjuring it and the Dementor. It is a positive energy force, a projection of the very thing that the Dementor feeds upon. Each Patronus conjured looks very different for each individual, each is unique to the wizards who casts it. "Expecto Patronum" is the incantation that conjures the Patronus. Adding to that, a single happy memory, one that really is very happy is needed. As the Dementor sucks the happiness and joy out of a person, such happiness, strangely, is also needed to strengthen the charm.
Only a wizard of great power, wisdom and strenght is able to conjure that Patronus without a happy thought. Instead, determination and strong will power could replace the latter.
