I think the intolerance of the left is due to two factors. Probably not limited to the left but currently surprisingly common on the left. Firstly is the idea that what they think reality is is what reality is, rather than matching reality to thought. Some of this is pure hubris of course. (The opposite of course has its flaws as well, as what we see isn't always what it is either), Secondly is the tendency for those on the left to respond emotionally. This certainly is not limited to the left but definitely some people are more emotional than others.
Naturally the adage "moderation in all things" should be considered. A healthy skepticism should always be maintained, that what you think may or may not be true and what you see may be all that is, or not. Only with skepticism can critical analysis be conducted. And the greatest enemy to skepticism is bias.
Regarding emotion, it adds value and quality to life. Like flavor and color. But it should not and cannot not replace what it is tied to, is not in itself value. And thus when one's idea is challenged, the idea has to be allowed to defend itself unarmored by emotions. Because emotion binds and bias the idea from truth.
Surprising though how some can imbue abstract thoughts with emotional values. Perhaps the surprise is really I, being able to compartmentalize and keep the two separate.
Thought for the post: when Descartes say "I think therefore I am" how can he be certain this is not just a close loop process of his programmed computer head rather him as a sentient and independent entity?