It’s easy to pick on pessimists, especially if you are not one. Besides, they expect it, right? (Sorry!)
There are some positives to being pessimistic. Pessimists tend to be better prepared for things going wrong, where as the hyper-optimist cannot be prepared for problems they though could never happen. Pessimists are also less likely to be derailed when “bad things” happen.
However, pessimism can cause a person to not even try, and sometimes we get what we expect, be it good or bad. In marriage a pessimist can annoy an optimist by seeming to always be down on whatever the optimist thinks or wants to try. The intent is not to say “you can’t do that” or “it will never work” but sometimes it comes across that way. If you are the more pessimistic, learn to word things in terms of “have you thought about what would happen if…”. If you are the more optimistic spouse, try to see the words of your pessimistic spouse as a warning, not a condemnation.