Well, there can be many different ways people see the term 'boring'.
But since you said our opinions, I'll just say mine.
It usually when someone doesn't see the funny point of view, and are always serious. I know you can get different types of humor. But I'm talking about when they don't smile, like at all. It makes me feel depressed when I come across this.
Rarely though, luckily.
But that's my opinion.
It's just mainly when people always have a dead-pan face look, and also it can happen if no one has the same interests. Cause then i usually find it hard to talk to them, especially if they're not a very nice person themselves.