Love is blind
Definition 1 of 1
Pro Tip 1/3Emotional Bias
The phrase implies a person in love might ignore the negative traits of their partner. 
She forgave him too easily, but then, love is blind.
Pro Tip 2/3Metaphorical Sense
Used metaphorically, 'blind' means lacking perception, awareness, or discernment. 
He never notices her shortcomings—love is blind, after all.
Pro Tip 3/3Casual Expression
It's informal and often used among friends or in casual conversations, not formal settings. 
They say love is blind, but I just think he's smitten.