Summarize this article in one sentence.
Find that person you know will be real with you, and ask their opinion. Often when we're in crush-mode we can't get a good perspective on the situation. Your friend may be seeing something you don't, and could save you a lot of heartache, or she could give you the extra push you need to make a move and ask him out. Guys who are already in relationships are sometimes extra attractive to single girls. It may be because one girl has already given him her approval as boyfriend material, so you know he probably has good qualities.  You may feel competitive with the other girl, or you may get a temporary self-esteem boost from stealing someone else's guy. Let their relationship run its course and only pursue the guy when he's single. Does he spend all weekend in a dark room playing video games while you prefer to be out hiking and camping in your free time? Do you have similar values? Being different doesn't have to be a deal breaker--you may end up really complimenting each other and learning about things you never knew you liked. Consider if your differences can be worked through. If part of his personality is that he's a huge flirt and that makes you uncomfortable, it might lead to major problems down the road. Ultimately, it's up to you to figure out whether or not you like this guy--you know yourself better than anyone else. Don't make yourself crazy or miserable trying to figure it out. When you feel something as intense as a crush, it can seem like the most important thing in the world. But remember there's more to life than this one guy--don't forget about your friends, your schoolwork, your family, or that there are tons of other guys out there!
Ask a trusted friend what they think. Back off if he's already taken. Think about whether or not you are really compatible. Trust your instincts and don't overthink it.