I quote Tommy Lee Jones in Men in Black: "A person is smart. People are dumb, panicky animals, and you know it." While not about sanity, specifically, similar applies here.
In general, a single person is relatively reasonable and sane. Get people together, however, views clash, and beliefs are tested. That's the recipe for "crazy stew".
We are all sane and reasonable when we want to be. The key is for us to treat each other as if we are sane, and do our best to be sane too.