There are those who only can feel good about themselves by putting other people down. I have never understood WHY that is. Do you?

Putting others down is a sign of insecurity.   Insecurity can stem from many things - having controlling parents, tamams, numerous experiences of failure and rejection, shyness, low self-esteem, etc.   No one likes to feel small (insecure).   If a person feels so small and insignificant long enough, they find that the only way they can make themselves feel big is to bring others down.  Once they realize that they can make people feel as low as they do, they feed off of it (it gives them a sense of power and control).  If not dealt with, it will become a "coping mechanism" for them to not feel so insignificant. Once this becomes set into a person, it is hard for them to get out.