Full Idea
No one can lead a happy life if he thinks only of himself and turns everything to his own purposes. You should live for the other person if you wish to live for yourself.
Gist of Idea
Selfishness does not produce happiness; to live for yourself, live for others
Source
Seneca the Younger (Letters from a Stoic [c.60], 048)
Book Reference
Seneca: 'Letters from a Stoic (Selections)', ed/tr. Campbell,Robin [Penguin 1969], p.96
A Reaction
It is important to see this as a key aspect of the ancient aspiration to virtue. The end result is not far from Christianity. It is simplistic to see the quest for virtue as a crass self-obsessed quest for self-improvement. We are social.