Stability, lower energy state and Octet rule.
Every system in this universe tends to stabilize itself and each one has a way to attain stability.
Like this, individual atoms are also unstable. To attain stability they form chemical bonds.
In nature every thing acquires a state of low energy.
Individual atoms have high energy, they lower their energy by the formation of chemicals bonds between them.
Another reason for chemical bonds formation is Octet rule.
According to octet rule "every atom tends to complete its valence shell with eight electrons by losing, gaining or sharing electrons".