The example for this is almost always given as "when you're
drunk" or the imfamous
beer goggles. Although I agree that this is a
condition that would cause you to see something as more
beautiful than otherwise, it is not the only one, and not one that I find
attractive and so I must offer my own interpretation.
Another time where a person would behave like this would be when he is in love.
When you love someone you cease to be able to see them in any objective manner because you do not see people with just your eyes, you see them with your heart as well, and when your heart becomes more involved, it affects how your mind interprets what your eyes take in.
I'm fond of saying that while personality can make up for a less than stellar appearance, appearance can never make up for a poor personality.