Because they trusted that person. Now they are not sure they can trust themselves or anyone else. The thing to remember is things get better with time, and even though they are hurting now they have learned something from that experience. The lesson may not be clear at first, but it will come.
Because they are stupid.They feel like they are the only one who suffering.But some others suffer worse.They think that everything in their life should be good,perfect ...and what not..but that aint happen in real life..
Whether u have a friend or family member that u "trust" & they let u down then of course that would upset that person but that type of behavior shouldn't discourage ppl from socializing & meeting new ppl--it'll b a disappointment for awhile, but they'll get over it.
I believe it's because something happens that they can't handle. Like bad things/situations that constantly continue to reoccure and it makes that person frustrated. Apparently things don't seem to be going right for them.