by的用法Count your age by friends,not years.Count your life by

by的用法Count your age by friends,not years.Count your life by smiles,not tears.这句话是不是有错误?by smiles为什么要加s?是因by后面要跟单数吗?那 friend为什么又加s?Count your age by friends,not years.Count your life by smiles,not tears 衡量自己年不年轻，看朋友，而不是看年纪。评价自己过得好不好，看笑容，而不是看眼泪。