It simply means that he doesn't want a relationship. I know that him flirting with you and all that gives you hope that he likes you but the fact is that if he really wanted to be your boyfriend, he would have been after you told him that you liked him. And since he isn't, him still flirting and being around you simply means that he likes you as a friend and think you're fun to talk to. Flirting doesn't always mean that they like you romantically. Some people flirt for fun or it's their way on getting to know someone.
Yeah, I experienced something very similar to this during my sophomore year of high school. I told him that I liked him and he said that he had to "think about it". He constantly flirted with me afterwards as if nothing happened and I had thought that maybe he was just being hesitant. But finally I learned the hard way that he really didn't like me that way and that realization was more painful than the actual rejection. This eventually became the first time I had my heart broken.
But trust me when I say that this really has nothing to do with you. It doesn't mean that you aren't pretty enough, smart enough, or cool enough. If he doesn't like you then it simply means he doesn't like you and that's perfectly okay. There really isn't much else to it and it won't be worth your time to change who you are in order to impress him because it never works. If he wants to change his mind, let him change his own mind. Like in my situation, the guy who I had liked sophomore year surprisingly changed his mind and did end up asking me out when we were seniors. Although it was a pretty awful experience and I wish I could take it back. Honestly, I don't even know what I was thinking for liking him back in 10th grade!
My suggestion to you is that if you are okay with just being friends with him like you are now, then remain friends and don't hope that something will happen. It'll happen on its own if it really was meant to be. But if you keep hoping and hoping that you'll get together just because you flirt a lot, then slowly start to stay away from him because this will only cause you to be frustrated that nothing is going the way you want it to go.
Anyway, I hope I wasn't too harsh about this. I'm only trying to tell you the unfortunate sad reality of how this situation typically works to hopefully save you from a more painful heartbreak and possible embarrassment if you were to find this out on your own. Just remember that this isn't your fault and that it's okay to feel disappointed about this. But don't let it get to you because there are surely better guys out there in the future who would love to be with you!