smile and the (whole) world smiles with you(; cry and you cry alone)

proverb People like to be around those who are happy (but not those who are sad or morose). A variant of "laugh and the (whole) world laughs with you(; weep and you weep alone)." I know he's still hurting from the breakup, but John's misery has made him really hard to be around. Like they say, smile, and the world smiles with you; cry, and you cry alone. His sister just has such a sunny, cheerful disposition that you can't help but feel happy around her. Smile and the whole world smiles with you!