Be me, complete the game in 100% all by yourself, feel like you want to help other players and enter to a coop server.
Get kicked in the first one
Get kicked in the second one
Get kicked because some fuckbois doesn't want help of another guy like others
Get in another server and bond with them but they have to leave...

It's a result of the way crossplay between the Steam and GOG versions has been implemented. The TL;DR version is that you can't make a private game and have crossplay and you can't set a password, so people make "open" games to play coop with just their friends.

Which means you'll likely get kicked from those games without warning. And then there's the chance of bog standard connection issues between you and the host causing you lose connection.

Exactly what was said above. I wasn't kicked out in a single game so far, but I have left a couple of games because the latency (and geographical distance) was too big, causing moderate freezes and then it causes bugs like your damage not translating to the host, etc.

Also it goes without saying but saying hello and asking whether the host is up for co-op with you is a good way to start. Observing their playstyle (whether they take it slowly and explore/rush/loot, or rush in instead berserk style following just the objectives, skipping everything else) and adjustic to it can help to stay in said game.

You can do it mate, I believe in you!

edit: Today I was kicked 5x haha, I surfed the wave of kickery for the first time. Hovever from those people only one had a decency to reply to greeting and said that he was waiting for hi co-op buddy and that they would like to play together undistrubed. I left afterwards. Anyway, internet & people, so it's hit or miss really. Had some great co-ops mainly during the weekend though. Met 3 guys from Brazil today and they thought I was from Mexico. Maybe you could play with em? Level range ~95. Seemed pretty chill to me...