Source, in full
Attributed to Plato in No Ordinary Moments: A Peaceful Warrior's Guide to Daily Life (1992) by Dan Millman. It has also been wrongly attributed to Philo. It is a variant of the Christmas message "Be pitiful, for every man is fighting a hard battle," written by the Scottish preacher Ian Maclaren (also known as John Watson) in 1897.
Notes from Wikiquote
Be Kind; Everyone You Meet is Fighting a Hard Battle. Plato? Philo of Alexandria?