Supposedly, there's something that a human brain (or, rather, some part of the human brain) tries to maximize (by expected over time sum of that something, … or something :) ) (as far as it can). Probably correlated with what some people describe as “subjective feeling of pleasure” (see “phenomenal judgments”).

A more interesting (and less certain) idea is: if a human considers that “some part” as own identity (“valued identity”) and all other parts of own brain as “external” (not more valued as other parts of own body), that human will seek out to be wireheaded (whether agreeing verbally to proposed thought experiments or not).