In general because of scarcity I'm likely playing med for people who have way more experience and play at a higher level than me and making mistakes is a part of the learning process. It's more that if I feel like I'm not going to be respected by the people on the pug or treated like a nonhuman that I'm not really going to be inclined to add, and allot of times it can be little things.
Examples:
If I add vs an invite medic and the invite medic gets picked hella early, it just makes me feel like my team isn't going to want me there because I'm so much worse than the other person and it will be my fault that we lose
Going into a pug making a mistake and Instead of being told something on how to adjust my gameplay by a well adjusted human I get a neet who's sighs audibly into his mic, rages, or stops talking altogether and gives up after one mistake.
Getting called shit on stream during the pug, like I'm not gonna hear about it right after. Why would I feel like playing with you if you do this.
People who troll, throw, but then get mad over any time a medic makes mistakes.
People who pick the friend team + the medic but then don't actually include the medic in their memes, so it's like you are left out of the pug experience.
Idk pretty much every medic I know has their blacklist of players and if that person is added and likely to be picked it significantly decreased the likelihood I add.