Full Idea
Behind the general praise for 'altruism' is the instinct that the individual will be best safeguarded if everyone looks after each other....it's the egoism of the weak that created the praise, the exclusive praise for altruism.
Gist of Idea
Altruism is praised by the egoism of the weak, who want everyone to be looked after
Source
Friedrich Nietzsche (Writings from Late Notebooks [1887], 14[5])
Book Reference
Nietzsche,Friedrich: 'Writings from the Late Notebooks', ed/tr. Bittner,Rüdiger [CUP 2003], p.241
A Reaction
I don't understand why Nietzsche so strongly despises the weak. Callicles (in Plato's 'Gorgias') embodies the strong, but he is utterly unlovable, and appears to be motivated mainly by a desire to have fun at other people's expense.