Why do guys take advantage of nice girls??

It seems guys like to take advantage of nice innocent girls.Is this true? I'm a nice girl, but I'm not stupid and I have a good head on my shoulders and I'm not a pushover.Yet guys seem to always want to "hook up with me" but not get into a serious relationship.A guy once told me that I seem very sweet and innocent and a lot of guys are attracted to that therefore making them want to sleep with me but that a lot of guys will want to take advantage of me.Why? What happened to the good old days when a guy met a nice girl he would treat her with respect and if she wasn't so nice then he wouldn't be so nice to her.It just seems like guys these days are just dogs and want nothing than to hook up with women (good or bad) and leave them and don't even care about how they feel.Is it a vibe I'm giving off?I often times have thought about this and it seems every time I think negative thoughts they come true!
Why do guys take advantage of nice girls??
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