Why silicon has great affinity for oxygen?

It would appear that the two 3p electrons in silicon can combine with the four 2p electrons in oxygen to complete the 2p orbital with 6 electrons, making it very stable. However it is not that simple. The two atoms make covalent bonds, sharing the electrons instead of delocalizing them. That is why you form SiO2, because the two electons in silicon's 3s orbital are also shared, allowing silicon to complete the 2p orbitals of two oxygen atoms.