I can't be the only one that gets a lot of satisfaction from completing something difficult with random people!? I'm in a raiding guild and we down content like anyone else but with that said it's almost expected due to you knowing how your guild could/should perform. Cheering or a slight “whew” is the height of our excitement and even then, it’s the first time you kill the boss and not so much after. But when you join a pug, it’s a lot of unknown factors where you almost expect to fail. So when they are successful, it’s almost equivalent to when my guild downs a heroic/mythic boss for the first time.
The downside of PUGs vs guilds has almost the exact opposite effect in a sense. When your guild fails at downing a boss, the disappointment multiplier is higher because the expectation is that your guild should be able to overcome this obstacle (even worse when you’ve downed it in the past). But when a PUG fails, meh, for me at least, I’m not even mad. Yeah, I wasted my time attempting something with randoms but I knew the risk going into it.
While I get why some people do it, I rarely lay into anyone in a PUG. The times I have, it was because I was told I was doing something wrong which might’ve been the case, but how the person said it triggered it. For the most part, I enjoy PUGs, even the failed ones. For every failed one, I can count five successful ones within a 3-4 hour gaming period.