Well, guys don't like girls that are clingers. Are you clingy? Guys might be interested in another girl. Guys might have girl friends. Are you sure he doesn't have a girl friend? Guys could be gay. Are you sure he's not gay? You might have turned him off somehow or he might just view you as a friend or want casual sex.
I will do my best to reply with an educated, humble and honest answer. Ultimately, it is up to you whether you want to listen to my advice or ignore it completely. Sometimes, my advice may be wrong; occasionally, it will be right. Regardless, I want to do my best to give people answers they are seeking.