Full Idea
The feeling of guilt, of personal obligation, had its origin in the oldest and most primitive personal relationship, that between buyer and seller, between creditor and debtor.
Gist of Idea
Guilt and obligation originated in the relationship of buying and selling, credit and debt
Source
Friedrich Nietzsche (On the Genealogy of Morals [1887], II.§08)
Book Reference
Nietzsche,Friedrich: 'On the Genealogy of Morals/ Ecce Homo', ed/tr. Kaufmann,Walter [Vintage 1969], p.70
A Reaction
In other words, lofty Kantian ideals started life in the grubby world of the Hobbesian social contract, and self-seeking has been disguised by idealism. Too harsh on Kant, who explains why contracts have force, not just convenience.