-Why are you being so nice to me?
-I figured you're not a bad guy.
You're just not a people person.
Meaning a person who is not polite with people?
No. A person who greatly enjoys the company of other people.
Not so much somebody who isn't polite, just a person who isn't gregarious or overly social.

A 'people person' is somebody who likes talking with everybody, loves being in groups of people, has a great raport with people, etc, compared to a person who may still be polite, but doesn't enjoy meeting new people, or working with groups of people.

Somebody who's a 'people person' is the person who always makes a point of introducing themselves to everybody in the room, is always meeting new people, shaking hands, talking, etc. They make an extra effort to be very social.

Somebody who isn't a 'people person' can still be polite, they're just a bit more reserved and quiet.
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