When you start ignoring reality. And I don't mean reality as in "all the bad stuff" like no, I'm talking about the good and bad. Being unable to tell if something is actually bad or ignoring it because you are being too positive can be dangerous. It's the same way with negativity, like you ignore the positive stuff so all you see is negative, that leads to problems.
You just need a balance, which is hard to do but try to at least look at both the positive and negative. There will be times where you are more negative than positive and such, but only having one is what can lead to bad problems.