He that is a friend to himself, is a friend to all mankind.
Notes from Wikiquote
"I shall tell you what pleased me today in the writings of Hecato; it is these words: 'What progress, you ask, have I made? I have begun to be a friend to myself.' That was indeed a great benefit; such a person can never be alone. You may be sure that such a man is a friend to all mankind." ["Interim quoniam diurnam tibi mercedulam debeo, quid me hodie apud Hecatonem delectaverit dicam. 'Quaeris' inquit 'quid profecerim? amicus esse mihi coepi.' Multum profecit: numquam erit solus. Scito esse hunc amicum omnibus."]