Be pitiful, for every man is fighting a hard battle.

John Watson (1850-1907) Scottish writer, preacher, theologian [pseud. Ian Maclaren]

The British Weekly (1897)

Frequently paraphrased "Be kind. Everyone you meet is fighting a hard battle." Frequently misattributed to Plato. More discussion about this quotation here.  See also a later expansion on the theme by Watson here.