Metals want to give up electrons to attain the next lower noble gas configuration. If there was something around that would take the electron, they would be perfectly happy to give up the electron and stay positive. Non-metals don't - they want extra electrons to reach the next higher noble gas configuration. So when two metal atoms are bonding together they are both trying to get rid of the electrons, and if the orbitals can overlap, the electrons can be delocalized into a generalized pool of electrons spread throughout the metal. When two non-metals bind together, they are both struggling for the pair of electrons, and the pair stays localized between them while they play tug-of-war for them.
Okay, that's a really anthropomorphic description, but it always helped me.