The original quote is from Shakespeare. "Love is blind, and lovers cannot see, The pretty follies that themselves commit." ~ The Merchant of Venice – Act 2, Scene 6. The meaning is that when a person loves another person, they fail to see the faults or imperfections of the person they love.
The best way to answer this is through a personal story: I fell for this guy first year at college. I thought he was cute and cool and fascinating and deep and sweet and basically every other positive adjective in the dictionary. I told him I liked him and got a metaphorical slap in the face when he told me he didn't like me back and began ignoring me. I'm over the initial begrudging him his reaction (he is entitled to like and not like whoever he wants to), but now I look at him and realize that . . . he is not really that good-looking and he is admittedly dorky and socially inept and flirts with every girl he sees and is a glory hound and just generally not the person I thought he was while I was in love with him. I was blinded by love, and couldn't see who he really was because of who I wanted him to be. There's a saying: "No one is perfect until you fall in love with them."