I don't think it needs to be at all.
I think the issue, regardless of whether or not you are an optimist, or skeptic, or pessimist about our prospects, is how belligerent you are in how you present your argument.
If you are optimistic about a prospect, you should have no issue at least having a discussion with a skeptic or a pessimist. Same if you're a skeptic or pessimist. Just because you think one thing, it doesn't mean that someone who is politely presenting another side of the argument is doing anything wrong. That's what a discussion is after all.
The problem is when people try to force their opinions onto others, and try to demean the other person's views as a way to "win" the debate.
People can be excited about whatever they want, or not. The only time it's ever actually an issue is when you get the types who respond like "YOU'RE CRAZY AND STUPID FOR THINKING SUCH A STUPID THING, OMG WHAT A BUNCH OF SCHMUCKS, YOU'RE ALL CRAZY, I'M RIGHT AND YOU'RE ALL WRONG, BUNCHA DUMMIES!"
If we all agreed on things, this place would be hella-boring. We just need to remember not to be d-bags about our differing opinions, that's all.