Harlow's subjects were miserable because they knew what life was like outside of the box, and they knew that other people existed, and they knew what human contact was like
take animals that have never been seen by humans. on isolated remote islands etc... they're typically not afraid of people, because they've never seen a person and they don't know if we're a threat or not (some animals in this situation are afraid of people but those animals are naturally overly cautious and are afraid of everything just to be on the safe side). they've never seen a person, so they don't know people exist, they know that the other animals they've seen exist so they've got time to ponder what to do about these other animals. this is the same situation as the guy in the box, he's never seen another person so he has no frame of reference as to what their life is like, or that they exist.
these kinds of thought experiments are hard, because you are forced to attempt to think outside of the framework of how you experience things and assume an experience profile that you have no basis for experiencing.
a good example comes from a conversation me and my brother had with my grandmother when we were young. my grandparents grew up with french (cajun) as their primary language, they were taught english in school. my brother asked my grandmother whether she thought in english or french (i had to clarify this "mais dat little lady inside you head. does dat lady speak french or english?")...it makes perfect sense that someone's inner dialog would be in their primary spoken language, but for someone who's primary language is different than others, it's hard to wrap your brain around the idea that someone else's thoughts are in an entirely different language than yours. which is doubly hard to grasp when someone is bilingual and even worse when someone is trilingual. Are their thoughts in their primary language? does their inner dialog switch between languages depending on the environment? if a specific concept is easier to grasp in a different language (it takes about 5 or 6 japanese words to convey a concept that only takes one english word) is it easier to think about that concept in a different language? my grandmother's answer was that her inner voice speaks in both french and english, but she couldn't elaborate on why it would ever switch between languages so i don't know if it was situational or random or what.
take colors. i know what orange looks like to me. when i see a color that's definitely what i know as orange i say "hey that's oragnge". but how do i know that what i know as orange is orange for you? i'm not talking about the naming of colors, i'm talking about the actual reception of the colors. we can both look at the same paint sample and agree that it's orange. but maybe if we switched eyeballs what you always thought was orange now looks like what you always thought was blue.