$$

Money.  The truest indicator of how someone really feels about you that there is.  Relatives, friends, enemies, coworkers- everyone.  Amazing how quickly people can turn into forgetful, snotnosed pricks when the issue comes up. 

My advice?  If possible, don't EVER borrow a dime, nor borrow to someone a dime, if you want to stay on good terms with the person.  I would rather give a $10 bill to a guy on the corner holding a sign, whom I don't know and DON'T expect it back from than give to someone I know is going to "repeatedly conveniently forget" to pay it back.  It is always in the back of your/their heads.

The shit is evil.  Just what it does to people alone is sufficient evidence.