The problem with wrangling a girl from another guy is that if she can be so easily persuaded to leave her boyfriend for someone else, what would keep her from leaving you for another guy? I would think twice about pursuing a girl who is dating someone else.
you kiss her
you don't if you do, both the boy and the girl will hate you (but mostly the girl)
If the girl is dating another guy yet showing interest in you, this implies that the girl is not trust worthy. there is no need to probe any further, don't go for a girl who changes dating partners.
It is difficult to divide her love. The best go and get your own girl.
You should wait for her to answer you if you have already asked her out.
You move on - this girl has made it very plain that she's not interested in dating you.
Nothing.....She already likes someone apparently otherwise she wouldn't be dating him...Get over her!
GO out with him unless he is already dating the other person
kiss her and tell her how you feel but don't say anything bad about the guy she is dating
If he is not dating the other girl as of yet then try getting together with him and tell him how you feel - you have to realize that it may not be the reaction you are hoping for as you already know he has interest in someone else. If he is dating her already it is best to let him be and move on.
Assuming that the guy in question is already dating you, then the short answer is because he is too immature to commit to one person.
its possible, but it might lead to major relationship problems if the girl that's dating the boy finds out