Why does she view me as a brother and nothing more?

I meet this girl named Emily about two years ago. The other day we were hanging out at the mall with a bunch of friends talking about marriage and what we would want from it and all. Then that night after dropping her home I started to realize that I truly had feelings for her (always felt something for her but not sure what. So later I call her up just too chat and randomly threw out in a conversation when we were silent saying "all the talk of marriage has gotten me wondering what it would be like if we were ever together. Then she asked why and I explained too her that I like her. And she said she loved me but doesn't see me more than a brother. We got so close begin friends that that's where it lead too (I think) so basically what I'm asking is, how do I get her too see me more than that? (a boyfriend) HELP PLEASE!!!!!