Saying that girls are lambs won't answer why.

It depends on the scene you are talking about. Generally, there are two kinds:

(1) As a girl of the opposite sex, say: You are my little sheep. Then you are the wolf, and she is the prey. Very emotional, very provocative, very frivolous.

(2) As a friend, say: You are like a little sheep. In other words, she is either quiet and obedient, or weak as a lamb with little resistance.