What does "I love you" really mean?

What does it really mean? A lot of my friends have started saying "I love you" to me and each other, but I don't get it and it bothers me. I had a boyfriend once (ex now, lasted a few months), and when I said I love you to him I always felt empty and guilty inside. It was too much to say to the guy I happened to be with when I was 15 (16 now). I didn't like hearing him say it to me and I don't like my friends saying it to me -- I care lots about my friends, but I don't "love" them. It's so strong. Isn't love supposed to be unconditional, and all that? If I did something to hurt one of my friends I wouldn't expect them to care about me. My little sister tells me she loves me sometimes, and I find it really hard to say it back, but at least in that case I mean it. I care about my friends and would do a lot for them, but I don't think that the same as loving someone. I usually just smile, look at my feet, and say "you too" or something like that. But then I feel off for a little while afterwards. It's like they're cheapening the phrase, when I think it should be worth more. I hate saying it and I don't get why they keep using it. Do they mean it? Does it mean less than I think it does? Should it mean less than I think it does? Am I just getting worked up over nothing? Is it weird that it bothers me so much?