One big reason is better than many little reasons.
A prompt, generous letter of thanks can seal a commitment which otherwise might disappear when the going gets rough.
You cannot make friends of your enemies by making enemies of your friends.
Moral outrage is the most powerful motivating force in politics.
Politics is of the heart as well as of the mind. Many people don't care how much you know until they know how much you care.
Make the opponents' goal more expensive than it's worth.