Theory...

To paraphrase a great quote, the problem with writing comedy is that it takes up all your time. Sitting at a desk and trying to come up with funnies is a lot of harder than it sounds. Even if you're a regular Oscar Wilde in conversation, when attempting to come up with comedy that will work in an isolated context in front of any number of strangers, you will experience no end of despair, exhaustion and general annoyance. Add to this the fact that you have to be able to create new material and perform old material regardless of what's happening in your life and how you're feeling and you're left with a fairly complicated art form (yeah, this is one of those needlessly pretentious posts).

When writing comedy for, say, a sitcom or a play you can think of a few jokes that arise from the premise and situation and then frame the story around them, you can focus on the narrative and hope the jokes bloom organically from it or you can do the same gag for 30 years. Ideally you will have a combination of the first two (and possibly the third, if all else fails). In many ways attempting to formulate a stand-up set is very similar. Ideally a theme will run throughout the set (although this isn't always apparent) and each gag/one-liner/anecdote will link back to it. Alternatively, you can deconstruct these conventions and create comedy from the pencilled in workings of a gag that you forgot to rub out.

Musical comedy, in my experience, often reaches the point where you have one brilliant line and a tune but no real structure. Instead of taking the Leonard Cohen route of writing poetry and then fitting them to music, I usually just come up with the one cracking couplet and a jangly chord sequence and graft from there. I'm sure that more accomplished musical comedians have a far superior working method to mine.

Essentially, the main point of this is that writing comedy is very hard and it's exceptionally easy to take good comedy for granted, so, if nothing else, have a think about the work that goes in to that knob gag or that bit where someone told a lie about a current member of government. Or don't. It might ruin it for you. I don't know.