It seems to me that some people enjoy learning more than others.
Is the difference at all linked to by pleasure responses in the brains involved? Do certain people get “high” off learning, and is it something you can become chemically addicted to?
WAG is yes, in that the amounts and transmission features of serotonin differ among individuals, and their release and uptake are triggered in different ways by individuals who may have a lower or higher threshold when certain activities are initiated or contemplated.
And that’s the technical explanation. 
Organisms with nervous systems seem to get “high” in some sense simply by stimulation of those nervous systems. The more complex the nervous system (i.e., the more advanced the critter), the more so this seems to be true.
Critters will do all sorts of non-intuitive things for stimulation. Witness amusement park rides.
It’s nice if that stimulation is pleasurable in any sense. But absent that, critters will do whatever it takes to get stimulation, even if it is gruesome. Caged animals, bored shitless, will often develop very self-destructive behaviors, including chewing on their own limbs until bare red meat is exposed and chewed up. Yes, I’ve seen it myself with a bored shitless caged sea lion. Any stimulation at all, even obnoxious, is better than none.
Apparently, for smart critters with enough brains, even mental stimulation will do. Witness mental stimulating tasks, even if they don’t actually involve learning new things – like doing crossword puzzles or reading pulp paperpacks. Anything that gets electrons jumping across those synapses seems to jazz critters. Chimpanzees and dolphins are well known connoisseurs of mentally challenging puzzles too.