You might repeatedly get into relationships that you expect to go one way and then they go another, or you could get involved with someone and then, as you become more intimate with them, you start to realize that they're really different from the person you thought they were. It's not that you purposely mislead yourself, but you do tend to build up romantic partnerships in your head before they play out, which is a sure prescription for disillusionment later. Why not simply read the signals as they come instead of getting caught up in your own little fantasy of how things are going to be?
Well, in short, because you're a romantic! That's what a romantic does -- they dream of romance, and they look forward to the day when they find that special someone who will be by their side 'forever.' But really, all of that is a fantasy, and the sooner you face that fact the better. You'll most likely find more than one special someone in your lifetime, and even if there is only one for you, there's no way to tell how the relationship is going to go until you live through it. Learn to embrace love's little surprises instead of deluding yourself as to what's possible or probable.