Yep. Because positivity is SO hard to pull off...
You don't have to pretend you never get upset, you have to avoid showing you're upset because when you do that, you're not using the chat for communication, you're using other players as punching ball to calm your nerves on.
If you are upset, it's already affecting the game, as you are prone to play worse than usual (less cooperation, longer reaction times, etc).
If you show you are upset, you're adding to that: time wasted interfering with useful messages, encouraging arguments with others to make them waste more game time not actually playing, and you can even trigger other players so everybody is upset.
It's not good for morale, it's not good for strategy, it's just a harmful way to relieve yourself from a burden.
So, what's left?
Being polite.
Being friendly.
Announcing your strategy/upcoming actions.
Proposing cooperation/asking for support.
Asking for advice (itemization, how aggro you should be on your lane, when to group, etc).
Praising good players.
If you have difficulties with the game boundaries, these examples are safe ways to properly use chat.