An idea usually starts with having an experience that triggers an emotional response- often anger, frustration, bewilderment, or more rarely, happiness. Then comes the hard part- they set about figuring how to communicate this to an audience of strangers in a funny way.
Sitting waiting for a bus may be boring, but "Waiting for buses sure is boring!" is not a great topic. But "do you think aliens wait for buses?", while not particularly funny, is a start. So, you write it down. Comedians have notebooks filled with things like "do you think aliens wait for buses?" Right now, my instincts tell me the idea is going nowhere, but you never know. 98% of a comedians notes will go unused on stage.
The 98% figure is the key I think. Want to write a great joke? Write 100 of them. One or two will be good. Probably.