Subject: Why does atom need to achieve octet or duplet in order to be stable?

Hi, I've been studying chemistry for a long time and have this one question 
pondering in my head:

Why does atom need to achieve octet or duplet in order to be stable? If so, why 
compounds/atoms become more stable after every bond (ionic/covalent) is made?

Or put simply, why does bond making is exothermic? Is it because when bonds are 
made, the electrons are drawn closer to the nucleus, so kinetic energy of the 
electrons are lost as heat energy? Therefore, this loss in energy causes 
the new compound to be more stable than its original atom. So, an atom will tend 
to make as many covalent bonds as possible. For ex: PCl5 is more stable than PCl3 
because it contains more covalent bonds?

Thanks in advance for your help and patience

Re: Why does atom need to achieve octet or duplet in order to be stable?