Case in point: today's strip.
Panel 1: We start off with John trying to grab on to an angrily reluctant and barefoot Mike. Why is the kid yelling "NO!!" and struggling so hard that he knocks Daddy's glasses off?
Panel 2: Since we see a pissed-off John holding a nail-clipper and telling his glowering little boy "Now, the OTHER foot", we learn that Mike actively resists trimming his toenails.
Summary: Given how hostile his parents can get, it's no damned wonder Mike responds in kind. The end result is to show us that the Pattersons are what Seinfeld called "tough monkeys".
ETA: For some reason, Lynn has put up another banner with snowmen.