Why do molecules form bonds?

Molecules are composed of atoms that have bonded and released the extra energy with them as bond formation energy.This bond can either be ionic or covalent.

But, sometimes molecules form coordinate bonds.This results in a complex, a compound formed due to coordinate bonding between two molecules.For ex, ammonia-boron triflouride complex

[#NH_3->BF_3#]

It is formed when nitrogen of #NH_3# donates its lone pair of electrons into empty orbital of boron in #BF_3#.This results in sharing of the lone pair between nitrogen of #NH_3# and boron of #BF_3#.

Sometimes two covalent molecules form a special type of bond called bridge bond and exist as diamers for example #Al_2Cl_6# which is a diamer of #AlCl_3#.

In bridge bonding of #AlCl_3# , the chlorine atoms of the molecules that are near to the aluminium of other molecule form a covalent bond with aluminium in such a way that looks like a bridge.

There are several other cases in which molecules form bonds and all of which I cannot discuss here.

In conclusion, molecules form bonds to achieve stability by filling up empty orbitals or by neutralizing charge as in hydrogen bonds.