The best answer is that this strip is quite a bit like the experience of knowing me. So I just write down things I think about or things I find interesting. The other thing is, the characters suggest storylines. This is more true in the newest strips than ever. They suggest their own actions and dialogue.
Character driven humor is one of the essentials of good writing.
When I learned this it improved my writing tremendously. Probably from his dad. The technical answer is that I just liked how it looked on him. Or Cher. 5. If they are, it's very casual.