Why everyone says that love is blind?

When you're in love with someone, you fail to see his/her flaws or imperfections.

"Love is blind" means that when a person loves another person, he or she fails to see the faults or imperfections of the person he loves.
It means that if you love someone then you can look past faults they have.

(idiomatic) A person who is in love can see no faults or imperfections in the person who is loved.

This was coined by Shakespeare and was quite a favourite line of his.

Modern-day research supports the view that the blindness of love is not just a figurative matter. A research study in 2004 by University College London found that feelings of love suppressed the activity of the areas of the brain that control critical thought.
love can make you ignore (or become blind to) problems or short comings of the person you're in love with