The controversial subreddits get a lot of attention (up and down votes) and so show up as hot/interesting threads, and have the most visibility. If you limit yourself to politics and news, then yeah, you'll see a lot of extreme comments and hotheads. But if you stop there and write off reddit as nothing but "toxic negativity", you've judge the 95% based on the 5%.
Go to, oh, r/bass (the musical instrument, not the fish), or r/rtlsdr (sofware defined radio), to name two I visit sometimes, you'll find pleasant, helpful communities. I imagine there are tens of thousands of other civil subreddits.
As a very small experiment, I guessed the names of a few subreddits that seem like they would exist and had a look around. r/poker, r/weather, r/airplanes, r/needlepoint, r/darts. 5/5, all nice communities.