Proverb
If Jack's in love, he's no judge of Jill's beauty.
Explanation
This means that a person is biased for if he is in love with another, her beauty would be great to him even if she were not pretty in the least bit.
Proverb
If you can not bite, never show your teeth
Explanation
Do not make empty threats: also used to warn against making a show of aggression when you unable to defend yourself.