नास्ति शत्रुः प्रकृत्यैव न च मित्रं कदाचन ।सुखदं मित्रमित्युक्तं दुःखदाः शत्रवः स्मृताः
In plain words
There is, by nature, no enemy at all, nor ever any friend; whatever gives happiness is called a friend, and whatever gives sorrow is remembered as an enemy.