I understand that a if a molecule has high electron density that means there high energy level, but why does that make them less stable and molecules...

I understand that a if a molecule has high electron density that means there high energy level, but why does that make them less stable and molecules with low electron density=low energy and more stability, shouldn't more energy make it more stable because of more electrons circulating in the atom. Also if possible can an example be made, so that I can better visualize the concept in organic chemistry term?