I think there's another aspect to wireheading specifically in humans: the issue of motivation.

Say you care about two things: pleasure, and kittens. When you work on maximizing the welfare of kittens, their adorable cuteness gives you pleasure, and that gives you the willpower to continue maximizing the welfare of kittens. You also donate money to a kitten charity, but that doesn't give you as much pleasure as feeding a single kitten in person does, so you do the latter as well.

Now suppose you wirehead yourself to stimulate your pleasure neurons (or whatever). You still care about kittens; that hasn't stopped being true. But now you are completely unmotivated to go maximize their welfare. If you're in a position where you can rip off the electrodes and go save a kitten that's stuck in a tree, you want to do this, but it may take an act of great willpower to do so.