Full Idea
There are three common accounts of amusement: superiority theories (Hobbes's 'sudden glory'), 'relief from restraint' (Freud on jokes), and 'incongruity' theories (Schopenhauer).
Gist of Idea
Amusement rests on superiority, or relief, or incongruity
Source
Roger Scruton (Laughter [1982], §5)
Book Reference
Scruton,Roger: 'The Aesthetic Understanding' [Methuen 1983], p.156
A Reaction
All three contain some truth. But one need not feel superior to laugh, and one may already be in a state of unrestraint. Schopenhauer seems closest to a good general account.