^You're right. We can't know what it's based off of, and relationships aren't always logical, but a huge case can be made for lust, not love.
Edward decided he loved Bella after doing nothing but watching her all night and listen to her say his name in her sleep. Bella is always going on about Edward's looks rather than his mind, or his sense of humor. This has more to do with the writing: SMeyer really could have taken out another mention of 'liquid topaz eyes' and 'pale, inhuman beauty' and tossed in a line about Edward's *brain*.
Bella and Edward sleep together after the first date— literally. I know she's a teenage girl, but there's a big point made about her asking if they can have sex, even though they've had less than ten conversations until that point (most of which went "What's your favorite color?" "Brown." "That's a nice color") and it was their first date— I just found that a little bit... meh.
All the times we read about them together, they're talking about their love and their future sex and kissing. Yes, couples kiss all the time, we all like getting physical, but I always wonder what *else* they talk about, other than their forbidden love. It must get boring to repeat the conversation over and over: "I want to be immortal. Kill me, please." "I'd rather kill myself than hurt you or take away your virtue." "Edward, please, I'm beginning to get frustrated. I want to die." "Bella, my love, I could never do that to you. Now let me quiet you with a kiss."
Sorry, I love writing fanfictions. I get a bit carried away sometimes.