Lewis Structure question

[Delilah Naranjo dis 3j]

Why does oxygen always need to have two bonds and why does carbon always needs to have 4 bonds? Can someone please explain this to me.

[Hannah_Chao_1D]

When drawing Lewis structures, you want the formal charge to be as close to zero as possible.
formal charge= (valence electrons of atom) - 1/2(number electrons in bonds) - (electrons in lone pairs)
Since O has 6 valence electrons, it wants 2 bonds and 2 lone pairs for a formal charge of zero. That is,
FC=0= 6 - 1/2(4) - 4
For C, it has 4 valence electrons so it wants 4 bonds. Using the above equation, FC = 0 = 4 - 1/2(8)