It is insane to describe someone's mind as "average." That implies 3 things: (1) what is the description of the mind qualifying the other as "average"? (in this case yours), (2) given a scale: (bad-average-good), what are it's patameters regarding intelligence?, and (3) can you really use an argument in the basis of qualifiers (describing yourself first) in order to describe the mind of others?
Your question has a negative correlation, you position yourself with a qualifier (let's say you are an "above average" mind) and that somehow gives you an abstract entitlement to impose a different qualifier to another mind? ("average" in this case). Establishing a system and giving both persons (you and the one you are judging) a value set regarding intelligence (accounting for a theory i.e.: Howard Garner Multiple Intelligences), we can say that you start with 12 pts and the other person with 17 pts. You place your qualifiers (let's say 'above average' = +1 and 'average' = -1): You "above average" (12 pts +1), Other Person "average" (17 pts -1). All providing a total of You = 13 pts against the Other Person = 16. Although you placed (or society as well) the qualifier of "average" unto the Other Person, he/she is still more intelligent than you (assuming the given values are real).
Since you are primed to do this (to assume that you are above average and the other average) a +1 and -1 resolution holds. Thus, you have a negative correlation: whereas you are upgrading yourself the other is degrading himself/herself, but not necessarily in proportion. You can exagerate your qualifiers ("above average" = +7, "average" = -3), and then, the co-variation of cause and effect (if X then Y, or, if more/less X then more/less Y) gets out of proportion, but according to the signs in your qualifiers (X is positive, Y is negative) then the casual relationship between your qualifiers has a negative character.
It is an insult to assume that your question has an already established resolution, and on top of that, to go ahead and ask why that happens. WHY does the 'average' mind act to fulfill his desires?
WHAT is fulfillment? WHAT is desire? (you should start with those)