Because he is emotionally immature and/or has emotional problems. Best stay clear of him.
It depends on the type of person he is really.
Find someone else.
Tell him you want him back and if he doesn't want you after that then find someone else.
Not necessarily. If they left you for someone else that would indicate they are just moving on - as you should.
As stated, he wants to hang out. Guys don't think overly hard about things, so what he says is what he means.
Go out with someone else
Would you want someone else to be messing with a relationship you have with someone...didnt think so.
First ask yourself, If he was flirting with someone else would I like it? Most guys that have feelings for you will not like you flirting with someone else.
simply tell the person you are in a relationship with that you are intrested in someone else and just be honest with them and tell them how you really feel.
Get out or stop.
DON'T. You can either wait for them to brake up or to sabotage there relationship. I suggest you wait but both would work. you should move forward in your life leaving behind someone who is in a relationship with someone else.It's useless to express your feelings to someone at this moment,when you know the answer would be a NO.
it is always better to have a relationship with your parents be4 someone else ypur parents are the people you can look up to when he/she breaks your heart :DDD