Why we basically group people into interesting and boring (or any other), can anyone share some of their experiences related to this?
In my own experience
I have found that I tend to make the judgements "interesting" and "boring" about people if I am actively looking to make friends.
Perhaps we choose our friends with a certain pragmatism in mind, a la Darwinian theory. We want our friends to hold to our own standards, so we create a selection process through which we choose friends who cause us to think those judgements that we deem "good."
There are many reasons to label a person just as there are many ways to make friends. In the end, we want to spend time with people who effect positive vibes. Constructing a dichotomy between interesting and boring is just one way to understand people.