Similarly to NancyLebovitz, I'd strongly question the assumption that reddit even is negative. Can you provide any good examples of this? Are you sure you don't just confuse disagreement or unexpected levels of directness with actual rudeness and pessimism?

Personally, I think reddit is one of the most friendly places I hang out on. The only sites that are even nicer and more optimistic are (more or less) rational learning communities, but that's really selection bias. Cynical pessimists don't tend to tackle hard problems.