In Are Wireheads Happy? I discussed the difference between wanting something and liking something. More recently, Luke went deeper into some of the science in his post Not for the Sake of Pleasure Alone.
In the comments of the original post, cousin_it asked a good question: why implement a mind with two forms of motivation? What, exactly, are "wanting" and "liking" in mind design terms?
Tim Tyler and Furcas both gave interesting responses, but I think the problem has a clear answer in a reinforcement learning perspective (warning: formal research on the subject does not take this view and sticks to the "two different systems of different evolutionary design" theory). "Liking" is how positive reinforcement feels from the inside; "wanting" is how the motivation to do something feels from the inside. Things that are positively reinforced generally motivate you to do more of them, so liking and wanting often co-occur. With more knowledge of reinforcement, we can begin to explore why they might differ.
CONTEXT OF REINFORCEMENT
Reinforcement learning doesn't just connect single stimuli to responses. It connects stimuli in a context to responses. Munching popcorn at a movie might be pleasant; munching popcorn at a funeral will get you stern looks at best.
In fact, lots of people eat popcorn at a movie theater and almost nowhere else. Imagine them, walking into that movie theater and thinking "You know, I should have some popcorn now", maybe even having a strong desire for popcorn that overrides the diet they're on - and yet these same people could walk into, I don't know, a used car dealership and that urge would be completely gone.
These people have probably eaten popcorn at a movie theater before and liked it. Instead of generalizing to "eat popcorn", their brain learned the lesson "eat popcorn at movie theaters". Part of this no doubt has to do with the easy availability of popcorn there, but another part probably has to do with context-dependent reinforcement.
I like pizza. When I eat pizza, and get rewarded for eating pizza, it's usually after smelling the pizza first. The smell of pizza becomes a powerful stimulus for the behavior of eating pizza, and I want pizza much more after smelling it, even though how much I like pizza remains constant. I've never had pizza at breakfast, and in fact the context of breakfast is directly competing with my normal stimuli for eating pizza; therefore, no matter how much I like pizza, I have no desire to eat pizza for breakfast. If I did have pizza for breakfast, though, I'd probably like it.
If an activity is intermittently reinforced; occasional rewards spread among more common neutral stimuli or even small punishments, it may be motivating but unpleasant.
Imagine a beginning golfer. He gets bogeys or double bogeys on each hole, and is constantly kicking himself, thinking that if only he'd used one club instead of the other, he might have gotten that one. After each game, he can't believe that after all his practice, he's still this bad. But every so often, he does get a par or a birdie, and thinks he's finally got the hang of things, right until he fails to repeat it on the next hole, or the hole after that.
This is a variable response schedule, Skinner's most addictive form of delivering reinforcement. The golfer may keep playing, maybe because he constantly thinks he's on the verge of figuring out how to improve his game, but he might not like it. The same is true for gamblers, who think the next pull of the slot machine might be the jackpot (and who falsely believe they can discover a secret in the game that will change their luck; they don't like sitting around losing money, but they may stick with it so that they don't leave right before they reach the point where their luck changes.
SMALL-SCALE DISCOUNT RATES
Even if we like something, we may not want to do it because it involves pain at the second or sub-second level.
Eliezer discusses the choice between reading a mediocre book and a good book:
You may read a mediocre book for an hour, instead of a good book, because if you first spent a few minutes to search your library to obtain a better book, that would be an immediate cost - not that searching your library is all that unpleasant, but you'd have to pay an immediate activation cost to do that instead of taking the path of least resistance and grabbing the first thing in front of you. It's a hyperbolically discounted tradeoff that you make without realizing it, because the cost you're refusing to pay isn't commensurate enough with the payoff you're forgoing to be salient as an explicit tradeoff.
In this case, you like the good book, but you want to keep reading the mediocre book. If it's cheating to start our hypothetical subject off reading the mediocre book, consider the difference between a book of one-liner jokes and a really great novel. The book of one-liners you can open to a random page and start being immediately amused (reinforced). The great novel you've got to pick up, get into, develop sympathies for the characters, figure out what the heck lomillialor or a Tiste Andii is, and then a few pages in you're thinking "This is a pretty good book". The fear of those few pages could make you realize you'll like the novel, but still want to read the joke book. And since hyperbolic discounting overcounts reward or punishment in the next few seconds, it may seem like a net punishment to make the change.
This deals yet another blow to the concept of me having "preferences". How much do I want popcorn? That depends very much on whether I'm at a movie theater or a used car dealership. If I browse Reddit for half an hour because it would be too much work to spend ten seconds traveling to the living room to pick up the book I'm really enjoying, do I "prefer" browsing to reading? Which has higher utility? If I hate every second I'm at the slot machines, but I keep at them anyway so I don't miss the jackpot, am I a gambling addict, or just a person who enjoys winning jackpots and is willing to do what it takes?
In cases like these, the language of preference and utility is not very useful. My anticipation of reward is constraining my behavior, and different factors are promoting different behaviors in an unstable way, but trying to extract "preferences" from the situation is trying to oversimplify a complex situation.