Ah, so that explains it:
[…] In other words, strong emotional ties to another person inhibit not only negative emotions but also affect the brain circuits involved in making social judgments about that person. The results, conclude Bartels and Zeki, suggest that attachment involves a push and a pull mechanism – you are pulled along by the strong sense of reward you feel when you love. But you are also pushed by a tendency not to objectively see faults in the other person which might threaten love, or put the brakes on, so preventing you rushing headlong into a relationship, because circuits responsible for critical social assessment and negative emotions are literally switched off. So love really is blind and there is a biological basis for the blindness. […]Posted by rv at June 15, 2004 04:46 PM to science!