The ghast are nether creatures, native to the soul sand valleys, and with a life-cycle than can be summarized in three stages.
The first one are the babys, after being born, ghasts are small, incredibly fragile creatures, that can't fly very high and on top of that, they have to fend for themselves in a hostile environment, to achieve this, ghasts are born with the ability of become completely invisible, something incredibly useful, however, after some time, their own bodies betray them, and as the system that will allow them to shoot powerful fireballs starts developing, baby ghasts will sometimes shoot by accident small fireballs, making them visible again for a few moments, and giving away their position to hostile creatures, only 60% of baby ghasts survive due to this.
The second one are the juveniles, once ghasts reach a certain size, they lost their ability to become invisible, but now they can shoot more powerful fireballs, and can shoot them at will, juvenile ghasts aren't big enough yet to not be a target for predators, and will use their fireballs to defend themselves, they can now fly to higher altitudes, but can't reach the top of the nether yet, juveniles are more aggresive that babys and adults, and will sometimes shoot other creatures just for being too close, they don't like each other very much. This is the shortest stage of a ghast's life
Finally, there are the adults, too big and powerful to have any predators, before humans and lagers reached the nether, the only danger they have to face was debris ocasionally falling from the ceiling, they aren't naturally aggresive towards other creatures, but can become aggresive if they see these creatures harming others ghasts, being that the reason for the hostily that most ghasts un their adulthood have towards humans.