There are things one can call negative that are really realism trying to save you from very bad decisions. See the negativity of telling the boss at Theranos that the tests don't work. I've seen many a startup fail in large part because they spent their runway unwisely, as cheap tests that showed that the existing direction had structural problems were just seen as negativity speaking.

There is such thing as negativity that will ruin your mental health, but the other side is just embracing delusion. We can see examples of successful people that keep embracing the delusion and succeed. But there are many that end up in unrecoverable positions after they burn themselves to the ground in said delusion. Around me, there's far more of the latter than the former.