Quotations about:
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Our anger and annoyance are more detrimental to us than the things themselves which anger or annoy us.

[Ὄγδοον, ὅσῳ χαλεπώτερα ἐπιφέρουσιν αἱ ὀργαὶ καὶ λῦπαι αἱ ἐπὶ τοῖς τοιούτοις, ἤπερ αὐτά ἐστιν ἐφ’ οἷς ὀργιζόμεθα καὶ λυπούμεθα.]

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher

Meditations [To Himself; Τὰ εἰς ἑαυτόν], Book 11, ch. 18 (11.18) (AD 161-180) [tr. Staniforth (1964)]
    (Source)

One of the points to consider when evaluating how others are behaving, especially when it makes us angry or aggravated.

(Source (Greek)). Alternate translations:

How many things may and do oftentimes follow upon such fits of anger and grief; far more grievous in themselves, than those very things which we are so grieved or angry for.
[tr. Casaubon (1634), 11.15]

Consider that our anger and impatience often proves much more mischievous than the provocation could possibly have done.
[tr. Collier (1701)]

What worse evils we suffer by anger and sorrow for such things, than by the things themselves about which those passions rise.
[tr. Hutcheson/Moor (1742)]

Consider, how much more we suffer from our anger and grief on those occasions, than from the things themselves which excite our anger or our grief.
[tr. Graves (1792)]

Consider how much more pain is brought on us by the anger and vexation caused by such acts than by the acts themselves, at which we are angry and vexed.
[tr. Long (1862)]

Consider that our anger and impatience often prove much more mischievous than the things about which we are angry or impatient.
[tr. Collier/Zimmern (1887)]

How much mroe unconscionable are our anger and vexation at the acts, than the acts which make us angry and vexed!
[tr. Rendall (1898)]

How much worse evils we suffer from anger and grief about certain things than from the things themselves about which these passions arise.
[tr. Hutcheson/Chrystal (1902)]

Bethink thee how much more grievous are the consequences of our anger and vexation at such actions than are the acts themselves which arouse that anger and vexation.
[tr. Haines (Loeb) (1916)]

How much more grievous are what fits of anger and the consequent sorrows bring than the actual things are which produce in us those angry fits and sorrows.
[tr. Farquharson (1944)]

The anger and distress that we feel at such behaviour brings us more suffering than the very things that give rise to that anger and distress.
[tr. Hard (1997 ed.), (2011 ed.)]

How much more damage anger and grief do than the things that cause them.
[tr. Hays (2003)]

The greater grief comes from the consequent anger and pain, rather than the original causes of our anger and pain.
[tr. Hammond (2006)]

Anger and the sorrow it produces are far more harmful than the things that make us angry.
[tr. Needleman/Piazza (2008)]