My theory on relationships, and how you know you've found the right one:
Before being in love, we have expectations. We think things will or should be a certain way when we're in love.
You might envision perfect dates of strolling down a street lit up by Christmas lights while sipping hot cocoa and talking about your childhood.
You might envision what your perfect partner will look like: good height, straight teeth, disheveled hair, dark brown eyes, stylish wardrobe.
You might be less shallow and think more about personality traits your future mate will have: caring, smart, funny.
In your mind, you build up a perfect mate and a perfect life together that follows, without ever meeting. Gambling on great expectations, you keep "your type" in mind, clear as a picture in your wallet. You are looking for someone who fits a list of characteristics you've created. You have boiled the love of your life down to a scientific formula. Then you find yourself out there among the other people somewhere around your age who are also single and looking.
That's when we date people and then break up with them for seemingly shallow reasons: their name has too many vowels or not enough consonants, the way they chew their food drives you insane, or they drive an Impala. It could be any innocuous reason, the point is the person isn't right for you, and you know it. Maybe they fit the scientific formula in every way, but you still find a reason to break up: you found a gray hair and bolted.
And then you end up dating someone not your type. Maybe friends set you up, or maybe you were friends with this person while you were dating others. Maybe you followed your mom's advice and "gave him a chance." He isn't like any of the other people you dated. If you saw him on paper, you would have immediately dismissed him as "not a fit." But it does fit. And suddenly, that characteristics list and "your type" is what seems shallow.
Because when you find the person who suits you, you know it. It's as if you just pulled on a tight dress and it hugs all your curves in the right places and makes you look better than you have ever looked before. This person makes you laugh and when you're not with him you think about what he's doing. He calms you down or gets you excited and love becomes more than a list of what you want. Your selfishness disappears as it becomes about the both of you being happy, together.
We make lists to tell ourselves what we want when we can't be completely sure what it is we're looking for. But you can't know what you're looking for before having seen it. Love is a connection: it's a perfect chemistry of two people that you can't reason away into a scientific formula. Instead, you stumble across it by chance: by pouring different liquids into beakers until two combine and make steam.