Answer:
Because of survival instinct.
Step-by-step explanation:
Before Humans became a social creature, they rely on their power to survive in the harsh environment. They fend for themselves, make their own shelter, fight off all the dangerous alone, etc.
Of course, many of them fail and died since there's only so much that a single human can do.
Knowing this, they try to combine their abilities and form groups to survive in the harsh environment. They soon realized that forming a group dramatically increase the chance of their survival since they can rely on other members to cover each other's weaknesses.
This is why humans started too transformed into a social creature.