Lately, I've have been acutely aware of how the mood of other people affects me too. No-can-do attitude is something that is hard for me personally tolerate. I'd probably need a lifetime worth of meditation in a cloister, before I could honestly accept the sort of hysterics that negative people seem to be able to cook up about nearly anything.

There would be far more sensitive and political ways to frame this, which goes to show that this effect is getting to me. As changing yourself always has a higher chance of success than changing the other person, my next task will be to find some tools to tolerate people less violently optimistic than myself. Again, an aggregation of tips from Google:

Avoid them

The obvious and best answer. Naturally also completely useless in most real cases.

Get rid of negative emotions

Lifehack! Really simple, randomly bumped into a YouTube-video about how you should just do some semi-hard maths in your head when getting emotional in a bad way. I'm going to trust this one, as it was presented by a guy somehow looking a bit like Captain Picard. So if I can't avoid a person, I might at least try to keep my own equilibrium and not get sucked into feeling annoyed and downed by them, by adding up some numbers. Merits a try, if nothing else.

Behaviorism

Reinforce positive behaviour, don't engage the negativity at all. Rant or venting merits an "Okay." A positive discussion warrants enthusiasm and congratulations. This actually sounds useful. I feel this is more about managing the other person and the relationship, than changing them into a more happy person, so it also fits the bill.

I actually have done this once for one person.

The person would often vent to me about everything and at some point I realized that I need to change the dynamic somehow. I let her/him know that if s/he had a problem, I would try to find a solution and suggest one. I stopped rewarding the vents with attention and comfort and just churned out more effective processes or clearer excels in a laconic manner. I offered nothing but solutions for a while and this actually stopped the venting quite fast.

Probably a trick you only want to try at the workplace though, friends and family might hate a person for using this on them.

If there are any other tips (or more thoroughly researched and proven lifehacks about quickly getting rid of negative emotions), please let me know!