Good question -- I think you're asking whether the feeling of happiness has to involve a contrast with its opposite. It's not a "problem" any of us will ever have! But here's an example to think about. In the movie Being There, Peter Sellers plays a character who feels happy all the time, because that pretty much exhausts his entire emotional range. That's just all he's got. The problem is, he therefore misses most of what actually goes on around him. So, he feels good all the time, without apparently experiencing any contrast. But I wouldn't trade lives with him!
This is a philosophical discussion. You can ask almost anything!
Ha ha! Yes, I often wonder what the take-away is supposed to be. I think the lesson here might be a couple of things. One is that we seem to do exactly what you say: we seem to focus on all the highlights in our imagination, but of course reality has a lot more than the highlights. So it's important to realize that that's sort of how imagination works; you have to be on guard against that if you want to be realistic about your plans. The other thing I'd point out is that scientific measures of happiness are, for the most part really, measures of the person's mood when they're surveyed. So the planning phase involves a good mood -- no surprise. But the actual vacation itself is, well, a week or two of living your life, and life isn't just a long string of good moods.
By the way, Jo, something similar happens even with money. People tend to overestimate how happy they would be if they were given money for free -- even as much as $1,000.
Daniel Kahneman makes a suggestion about vacations: for your next one, imagine that at the end all of your photos would be erased & you'd forget the whole thing. How would you plan your vacation then? You'd probably spend less time rushing around & making sure you got in all the "obligatory" sights. And you might actually have a much better time. Planning all the stuff you're going to do on vacation might be really fun. But actually running at the pace you plan for yourself can make a vacation a lot less fun.
Being ready to say no to the next $1,000 is a good place to be.
I've wondered about this myself a lot lately -- why is happiness discussed so much these days? I don't have an answer, just some guesses. I like your suggestion -- I suspect we definitely do talk about different parts of our lives in public today that were kept much more private in earlier times.
In the literature, it seems to have become just a truism that increased wealth in the modern world has allowed more scope for talking about happiness. The funny thing is, right after someone says that they'll go on to talk about Jeremy Bentham or even Aristotle; so, evidently, it's been a pretty big focus all along. My own guess -- which is not very inspiring -- is that that's where a lot of the grant money is these days in the social sciences.
I wouldn't say they're the same. A person could think that a life that isn't very happy is still no worse than they deserved. Even so, I would say that contentment is very important to happiness. When desires start taking on a life of their own, that's when we really stop enjoying life, and I think it's also when we start making a lot of bad choices and bad trades. Incidentally, this idea was fairly central to some very diverse schools of thought in ancient Greek philosophy.
I think people have always known that money can't buy happiness. The pioneering economist Adam Smith was really clear on this point. Still, what money can do is give you back more of your time so that you can have a wider range of options for how you spend it. So, why not just take it from here & put it over there? Well, obviously, that's a huge issue. I'd just point out that when people make money, they do it so that I can do what I just said -- they do it so that they'll build up future options. I'm not sure it would make for a happier society to take those options away.