Full Idea
The form of universality is itself the determining ground of the will, …and from this limitation alone, and not from the addition of any exernal drive, the concept of obligation arises to extend the maxim of self-love also to the happiness of others.
Gist of Idea
Universality determines the will, and hence extends self-love into altruism
Source
Immanuel Kant (Critique of Practical Reason [1788], I.1.1.§8)
Book Reference
Kant,Immanuel: 'Critique of Practical Reason (Third edition)', ed/tr. Beck,Lewis White [Library of Liberal Arts 1993], p.35
A Reaction
This is the heroic and optimistic part of Kant's philosophy, the attempt to derive altruism from pure reason. The claim seems to be that maxims don't motivate until they have been universalised. I fear that only altruism could add such motivation.