I think this is one of the nastiest things Dogbert ever said to Dilbert.
He is saying that Dilbert is a member of the "worse-off reference group" that other people compare themselves to, to make themselves feel better.
And what makes it really bad is that Dilbert accepts that his purpose in life is to make other people feel better by being worse-off.
I remember studying this theory in religious studies and psychology.. it's a bit depressing in itself! I think it's too simple.
Also want to point out Dogbert wagging his tail in amusement in as he makes that last point