What makes people hard-hearted is this, that each man has, or thinks he has, as much as he can bear in his own troubles.

[Was die Menschen hartherzig macht, is Dieses, daß jeder an seinen eigenen Plagen genug zu tragen hat, oder doch es meint.]

Arthur Schopenhauer (1788-1860) German philosopher

Parerga and Paralipomena, Vol. 2, ch. 26 “Psychological Observations [Psychologische Bemerkungen],” § 325 (1851) [tr. Saunders (1890)]
    (Source)

(Source (German)). Alternate translation:

What makes a man hard-hearted is this, that each man has, or fancies he has, sufficient in his own troubles to bear.
[tr. Dircks]