Can someone fully explain Foley's dislike of Mayo?

It's clear up to the point where he discovers his hidden shoes and buckles scam that Foley doesn't approve of Mayo, and when he's PT-ing him (or whatever you call it) he tells him that he isn't officer material because he doesn't give a sh*t about anyone but himself and how he lacks character. Mayo is a little bit of a loner (shown in the bit where he sits againsts the wall after finishing the assault course instead of cheering people to the finish line like a couple of the others are doing), but what exactly do people mean when they go on about character? Mayo wants to be a pilot, which isn't the same as being in charge of a infantry platoon, don't you just need the right abilities to fly an aircraft? So what if you're that much of a people person?

In WWII not every pilot was an officer, some RAF pilots were just Sergeants, isn't one of the main reasons a pilot has to be an officer nowadays something to do with the sheer cost of a modern military aircraft?