When we're not in love we are looking for an idea, the character we built in our minds. That person does not exict and will never exist.
Once you're able to let go of this and just have fun with people from the other sex, feeling relaxed in each others presence. You start to see that which you already have is perfect in it's own right. The person you're with is already perfect.

Now that I'm in love I start to notice that the above things just went unconciously. We just wanted to be together more and more often.