I think some people get so insecure about being mistreated in the past that they become hardened and bitter and decide to look after numero uno by any means. Others are just selfish because that is how they were raised and they expect special treatment and often get it. Others have mental illnesses which lead to them lacking empathy and understanding of exactly how their actions impact others. Some see how mean people seem to get everything they want and be treated better in life so follow them in the belief of treat em mean to keep em keen and it pays off for them to an extent. These people need assertive others around them who don't allow them to use and manipulate because if something keeps paying off for them why would they quit.
if a friend uses you in a not so nice way, they weren't really a friend to begin with. when you use someone, it's for personal gain... some people are so into themselves. they don't care so they don't feel anything... except for themself.
U use this term so readily.. I am feeling used again.. right now!
Some people enjoy doing things that are taboo or not in good conscious.