A relationship starts when you choose someone and they choose you back, above all other people.
To use my favorite quote from the book Microserfs,
and when you meet someone and fall in love, and they fall in love with you, you ask them Will you take my heart stains and all? and they say I will, and they ask you the same question and you say, I will, too.
Someone asked once, "Is there anything about your wife that you don't like?". She was SHOCKED to find out that the answer was yes. She thought you were supposed to love everything about the person you marry. I don't think you could find someone with no annoying habits if you searched for eternity.
The whole point is that you love this person so much, you want to be with them in spite of all their quirks. And when the going gets tough, or the quirks get quirkier, you remind yourself why you're together in the first place, because none of that little stuff matters. What is important is how you feel about each other.
But it doesn't end there.
You have to keep choosing that person again and again.
You have to make the effort to keep yourselves close together, especially when its hard. There are times when you havent slept in days or you're having money problems or work problems or you're just having a bad day.
And its on those days that its most important to make that choice.
Forget about the fact that he lost his wallet. Again. Overlook the fact that she burned your steak and the potatoes are under cooked.
Remember how it feels to dance in the moonlight. Bask in the glorious fact that you both love skinny dipping. Watch the movie Accepted for the 112th time so you can laugh until you both have tears in your eyes.