If you make yourself an ass, don't complain if people ride you.

人善被人欺，马善被人骑。

活学活用：ass

ass in grain

十足的大傻瓜

play the ass

作糊涂事; 出洋相

an ass in a lion's skin

气势汹汹的胆小鬼; 色厉内荏的人