The world is not so linear.
There are numerous possibilities, from just potential social 'unpleasantness/snootyness' to certain kinds of mental illnesses.
Never be to quick to judge.
Completely agree with this.
I have a dear friend who lately has started talking incessantly about herself without even realising and after pulling her up on it we've deduced that it's simply from her being stressed lately and worrying about a multitude of things, so verbal diarrhoea is her side-effect of coping. So I'm to pull her up on it when she does it as she always does it without realising.
eg.
She says: How was your day?
I say: Yeah, I had an interesting chat with my boss today about Holden stopping production in Australia -
She cuts me off: I had a convo about *insert something completely regardless here* with boss too. My boss told me XYZ and then this affected my mood in ABC and it reminded me of QRS etc etc etc...
The difference with some people, is that it becomes so habitual that they don't ever evolve past that way of thinking and consequently stay in that frame of mind where they feel most comfortable - talking about themselves.
Just because someone may not be as comfortable in their own skins as you and therefore talk too much doesn't mean you should instantly disregard them. I mean, if you really have no desire to network, learn from other people (even if it's how you'd prefer not to act/think) make new friends and enjoy moments of being a social creature when out and about with friends and acquaintances, then don't bother and leave.
But otherwise I see this situation as an opportunity to practise your own public speaking and persuasion skills and guide the conversation towards your preference. You can only ever control your own actions and emotions - so guide the convo using your charisma and talent with words.