Brushing teeth before going to the dentist has the same motivator as shaving pubes before going to the gynecologist: If someone's going to be looking inside there, you want it to look nice.

If you don't brush before going to the dentist then I'm going to assume you're a sociopath

I mean, I schedule my dentist appointments at the end of my work day. So I guess I'm a sociopath.
- You can take a toothbrush to work...
I go to the dermatologist every six months and they have to look at my entire body every time, I take extra care to be clean and smell neutral or faintly nice. Unless that one time I had to go by bike bugy that was outside my control. I feel pretty self conscious at those times.
Same goes for the dentist, I take extra care weeks in advance when I know an appointment is coming up. If you have stuff in between your teeth a brush won't reach, one quick brush before the appointment has as much effect as taking a breath mint.
Why? I'm paying for them to do it. You don't crank one out before getting a happy ending.
- I think a closer analogy would be more like "I don't shower before I ask a hooker to tongue blast my fudge machine"

also got to look pretty for the colonoscopy appointments

If you don't prep right you're basically wasting their time

I always assumed it was helpful to get the gunk off of your teeth so the hygienist doesn’t have to spend an extra minute or two with you. The less time I’m forced to keep my mouth open, the better. My lips get chapped, my tongue goes dry, I’m bored. It serves utilitarian function

I completely agree. That said, a little lip balm while you're in the waiting room can help with the chapping.

Unethical lifehack: if you start flossing every day about a week before your appointment, you'll be indistinguishable from someone who actually flosses regularly.

Unethical because you are only hurting yourself by not doing regularly anyways?
I should floss more often...
- Unethical mostly because lying bad. Lying to medical professionals bad and dumb.
  I only did it like twice. I floss regularly now.

I legitimately have a meal before going to my hygienist appointments, the more random small bits it has like seeds or tiny nori stripes and whatnot, the better.

Supposedly makes their job more interesting when they have something to dig for.

Their words, not mine.

You don't have to shave your pubes as a woman. Or man (or however you identify).

I was not expecting a compliment on my manscaping when I went in for my vasectomy.

Edit to add: I did not shave for today's colonoscopy.

I also floss for the week leading up to the dentist lol

Good advice bro

Do y'all brush and floss twice daily?

Brush twice, floss once per week or so. Dentist said to floss only once a day, I just don't do a good job of keeping up.
- My dentist says most people floss so badly that they push bacteria into their gums, so he prefers it when people don't floss.