Thoughts in the Night

Last night I was thinking why do people intentionally hurt people and does everyone let you down at some point or another?
I can't figure out why people do things on purpose they know will upset someone. If you promise something, keep your promise. If you say something, do it. Why deliberately not do something and hurt someone? Or why hide something that will eventually be found out? Isn't telling the truth now easier than the person finding out later? I was speaking to my friend Justine about this one day and she said this "people hurt people to see if they'll come back, to see if you'll give then another chance, to see if you care enough". Is that true? I don' really know but that is the best answer I've found in the past few years.
As I was reading my book for Discover New York, one of the people in the book said "everyone always lets you down at some point". I have come to realize that although not everyone has yet, they probably will. It might not be major, but it will happen. I think the thing you have to do if figure out who is worth it.