Why is saying "I love you" such a big deal?

I'm a person who isn't all that emotional, for some situations you consider normal I'm pretty cold and I never seem to understand them. This summer while I was drunk, I told this chick I love her (we knew each other forever) and she freaked out. 2 months ago she finally said "hi" for the first time after that evening. I was really confused, it didn't seem that big of a deal back then and it doesn't seem like a big deal now either.

Clearly you people will think I'm weird and, the hell, I maybe am, but still can somebody explain me why are people so freaked out they hear the phrase "I love you" ? Afraid of commitment maybe? I wouldn't know, I'm only 17 so take that into consideration as well.

Oh and, if I may ask, please don't downvote just because I'm confused and different from the others thinking I'm a weirdo or something, it won't help me get my answer at all.