We constantly have spit in our mouth 100% of the time but if we were made to drink any amount of our spit it'd be disgusting. Why?

It's the quantity. And also, by the time you've filled a cup of spit, it has cooled and dried a bit, maybe even breaking itself down, resulting in an increased viscousity, giving it a very unpleasant mouthfeel.

I get that the OP asked a question, but did you have to answer it with this level of detail?
- I can't help it. When someone asks a question, I spit up an answer

I feel the same way about cum that isn't directly from the "tap." Somehow it's fine from the source, but you put it in a shot glass and suddenly it's disgusting.

I feel the same way about breast milk. From a glass? No thank you. From the tap? Yes please.
- This doesn't apply to excrement, does it?
Huh. What about piss? Like if you pee in your own mouth directly, is that less disgusting?
- Still disgusting, but a bit less. Still pretty revolting though!
- That’s fine. Also fine if another dude pisses in my mouth. Or a bunch of other dudes.
  👅
I love furries and their sexpositivity. This comment made my day.

Same reason you wear your socks the whole day but once you remove them they become eww. Human brain is weird in that regard. Probably has evolutionary reasons but from an entirely rational POV it's weird.

I had some oral mucositis that caused some excessive saliva production. There was one time I accidentally swallowed a mouthful of it, it immediately caused me to vomit.

spit up
I've swallowed a mouthful of spit a couple times. Rather unpleasant experience.

Thanks...now I'm acutely aware of the spit in my mouth and when I swallow it.

Fluids that exit the body is like this. Only exception is blood from your own kin and tears. Anything else gets very dirty once out of body. Sweat, urine, vomit etc

Spit in a cup and drink it and watch how your peers react.

Are you drinking your kin's blood and tears?
- No - but wiping it off is no issue. If you drool on yourself I’m not gonna touch it

I swallow my own spit all the time. It's not disgusting

When you take a drink, there's backwash into the drink, and most people finish their glasses. And they don't feel like it tastes any worse.

If you have trouble finishing off your own drinks, it's a psychological issue not a physiological reaction to your own saliva

I think on this it would more mean, if you had to fill a cup with your saliva only, then drink it, there would be aversion to do so. However if it stays inside your mouth the whole time as regular saliva then the aversion wouldn't be there.
The question comes down to, why the sudden aversion to drinking it?
- maybe you should put your spit into a sodastream and carbonate it
- In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be extracted), enzymes (such as lipase and amylase), and antimicrobial agents (such as secretory IgA, and lysozymes).[1]
  I think it's mostly psychological, but if I had a cup of my own spit, it wouldn't last very long, it would quickly start to break down. And that would be unpleasant.
  So I think as much spit as I could fit into a cup in 15 minutes sure I could drink that no problem. Any longer than that and then you start having to worry about the breakdown and that's going to change the flavor.
- I mean... picking your nose is the same idea. It's the same thing for why you'd want to drink flowing water, not stagnant water.
  And some of us do our best not to backwash.
Eww backwash

You are swallowing that saliva during the day. Usually we make 0.5 and 1.5 liters daily (for us measurements 2 to 4.2 cups). And that needs to go somewhere. And it is very important part of our oral hygiene too.

So in metric the range is 3x and in imperial is 2.1x?

Freshness?