One-upping to such a degree, involving, caused by, or of the nature of a mental disease, where an upper is incapable of ever stopping and must refute any up of a fellow upper, even if the up makes no logical sense at all.
Regular One-upper: Dude, i bought a Jaguar last week. It's sick.
Pathological One-upper: Nice. I got an Aston a few days ago...
Regular One-upper: My Dad's getting a Bugatti Veyron today!
Pathological One-upper: That's nothing. For that much money you won't even get an apartment in my building. I have six.