
Laughter naturally comes as thunder comes - suddenly. That is the very mechanism of a joke, any simple joke. Why does it make people laugh? What is the psychology of it?
It builds up a certain energy in you. Your mind starts thinking in a certain way as you are listening to the joke, and you are excited to know the punch line - how it ends. You start expecting some logical end - because mind cannot do anything else but logic - and a joke is not logic. So when the end comes it is so illogical and so ridiculous, but so fitting, that the energy you were holding in, waiting for the end, suddenly bursts forth into laughter.

Finally, a range of studies repeatedly indicate that laughter is generally contagious. I advise you to catch ilness of joke.