Variant: you can measure the size of a person by what makes him or her angry 
Source: How to Win Friends and Influence People
                                    
“Real compassion lies within our ability to remember that an angry, vengeful or hateful person is usually just someone who can no longer bear the weary weight of his or her own carefully concealed despair.”
The Secret Way of Wonder
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Guy Finley 23
American self-help writer, philosopher, and spiritual teach… 1949Related quotes
                                        
                                        Original: Prima di mancare di rispetto ad una persona corretta, pensaci bene. Una persona corretta, tale gesto lo ricorda per tutta la vita. 
Source: prevale.net
                                    
                                        
                                        The Philistine magazine,  August 1901 http://books.google.com/books?id=xxI8AQAAMAAJ&q=%22There+is+something+that+is+much+more+scarce+something+finer+far+something+rarer+than+ability+It+is+the+ability+to+recognize+ability+The+sternest+comment+that+can+be+made+against+employers+as+a+class+lies+in+the+fact+that+men+of+Ability+usually+succeed+in+showing+their+worth+in+spite+of+their+employer+and+not+with+his+assistance+and+encouragement%22&pg=PA87#v=onepage 
"The Crying Need", in A Message to Garcia, and Thirteen Other Things (1901),  p. 163 http://books.google.com/books?id=iSo3AAAAIAAJ&q=%22There+is+something+that+is+much+more+scarce+something+finer+far+something+rarer+than+ability+It+is+the+ability+to+recognize+ability+The+sternest+comment+that+can+be+made+against+employers+as+a+class+lies+in+the+fact+that+men+of+Ability+usually+succeed+in+showing+their+worth+in+spite+of+their+employer+and+not+with+his+assistance+and+encouragement%22&pg=PA163#v=onepage
                                    
Country Living and Country Thinking, reported in Bartlett's Familiar Quotations, 10th ed. (1919).
“Truth lies within a little and certain compass, but error is immense.”
Reflections upon Exile (1716)
“Now I am weary and I can no longer tell good from Evil, and I need someone to show me the way.”
                                        
                                        Orestes to Electra, Act 2 
The Flies (1943)
                                    
2013, Brandenburg Gate Speech (June 2013)