If  you are biased about something your judgement does not work properly.

This reminds me one old Nepali Story.

One man in a village had two wives. He was very critical of his first (jethi) wife but he used always praise his second (Kanchhi) wife. One day when he was having his dinner he shouted his first (jethi) wife "why to  do always make rice so "Gilo"?  Then his  second (Kanchhi) wife replied from the other corner of the kitchen, it was me who cooked the rice today. The man then said...that's why it is delicious even  it is "Gilo".