Because there’s vomit on his sweater already!
Being a comedian or public speaker can be nerve-wracking. Whether it’s trying out a new bit or topic, or performing a new (or larger venue) the feeling coursing through your veins is indescribable.
I mean, I’ve tried: Excited? Stressed? Rushed? Electric? Who knows?
If you’re like me there’s this cycle I go through before I go on stage. It’s usually 1) Doubting the material 2) Positive Affirmations 3) Hyperventilating/Pacing 4) Wanting to take a nap 5) Then having my heart race before

What I’ve learned is that you have to go back to the think you worked on. You’re always going to think your stuff is not good enough. This is especially true if you peek from behind the curtain to see a restless crowd, or if someone goes on before you and does exceptionally well.
Realize you’ve been booked/asked to appear. The audience is already going to be receptive of you on some level. Your work will eventually have to send them to that entertainment promised land, but you get some time in the shallow end at first to find your footing.
“Kid, you got chops. Now it’s time to show the world.”