I think love is something that grows, the more you get to know the person. And romantic love, in my opnion, is the toughest to define. Love for your friends is easier, but it's the "true love" that confuses people. For example, my relationship with my husband. I have known him since I was 14, and I had a crush on him back then....I was sure I loved him. Got over that fast....mainly because I was young, and I didn't KNOW him....I saw the physical, but not the mental and emotional sides of him, which are very important. Seven years later, we had developed quite a good friendship, gotten to know each other a lot better, had the same goals and plans, and along the way, we realized we had fallen in love. I think if it were any less than that, it would have faded after a while.
I think it is certainly different for everyone. I know a girl who met this guy, and she could not stand him at first...he was loud, obnoxious, and not that attractive. But, the groups they hung out with got together a lot, and they got along better, and soon she realized he was funny and had a great sense of humor, caring and considerate, nice, and kinda cute. She realized she had feelings for him. It's that sort of thing that makes me believe that love is not immediate...like all things in a relationship, it takes work. Maybe not hard work, but more of a "continuous effort" sort of thing. You learn as you go with some of it, and after a while, experience helps you along.
But once you are there, it gets easier. I can't imagine loving anyone else the way I love my husband. But like I said, it wasn't immediate....I thought it was when I was 14, but as I got older, I realized the difference between love and infatuation.
Ok, now I'm sort of rambling, but I think some of this has made sense...maybe not. :)