Why would a girl talk to you, and all of a sudden drop the boyfriend bomb on you?

me and this girl have been talking for about 3 months, and to make a long story short, i just found out that all of a sudden shes now in a relationship with someone, why do women lead you on, then all of a sudden they get into a relationship with somebody else, do they not care about how they make us feel.Why would a girl talk to you, and all of a sudden drop the boyfriend bomb on you?
We don't always know when someone is interested in us as they don't always give us the right signals or tell us they are interested in us so if all you have been doing is talking and not saying the right words how was she supposed to know.Why would a girl talk to you, and all of a sudden drop the boyfriend bomb on you?
Women do that a lot, and to them its not a big deal. So the next time your talking to a girl, have the expectation that she's talking with other guys.That way if she ends up going out with some other guy, you won't be so dismayed. Also you should try to show some signs that your serious as you continue to talk, she may have thought you were just a friend.