People choose to play with other people's feelings because to them they are just playing a game and they just want to see what will come out of it. This is so because they are not actually expressing their true feelings. This happens when you get someone to believe in you and do what you want but instead of following through you do the opposite.
Because although their are alot of great people, some people just suck. Forget about them. Just move along to the next, then the next. Until you find someone kind, considerate, and respectful. Goodluck.