Answer:
To increase our survival chance.
Step-by-step explanation:
In order to survive in the harsh environment, Humans generally need to do 2 things : fulfill our basic needs and defend ourselves from external threats.
Both of them would be much easier if Humans are working together in a group.
For example, during hunter/gatherer eras, catching animals that can run faster than humans will be much easier if we have teammates to surround the prey rather than working alone.
On top that, being in a group also provide better protection from the attacks of wild animals or other humans. This is why human creatures get transformed into a social being.