A very old saw. Essentially what it means is that there is no rhyme or reason when it comes to affairs of the heart.

Love is blind. This is the reason why the girl you've had a crush on since Freshman Orientation is going out with some pumped up jerk who treats her like shit instead of you. It's also the reason why you have no clue what your friends are trying to tell you when they say that your new girlfriend is a possessive psycho-bitch.

If, six months after you've broken up with someone, you look back at your relationship and think to yourself, Wtf? What was I thinking?, your answer is this: Love is blind.