You're only looking at a single dimension.
You have problem X. Briefly, you experience what it's like not to have problem X, and it feels like "paradise" to be free of that problem. You imagine everyone who does not have that problem to be in paradise all the time.
But the paradise that they inhabit is only a paradise in that single dimension. All those people have their own problems (Y, Z, A, B, C, whatever) and those problems make their life hell. You don't have some of those problems, whatever they are (eg. cancer), so, from their perspective, you are living in paradise.
But I don't believe that everyone experiences the same amount of pain, or that life sucks equally for everyone. Some people are happier than other people, and it's because they have fewer problems. If you have problems X, Y, Z, and someone else only has problems Z and A, then they are relatively happier than you are. Problems Z and A still make that person's life hell, and they might be very unhappy, but that doesn't mean life can't get worse. (Trust me, it can always get worse. Pain is an infinite resource.)
All pain is real pain and everyone's pain deserves our respect. Even if another person's pain seems trivial to you, it is still a real pain, it exists objectively in their experience, and they deserve our sympathy. Unless you have a specific problem yourself, you have no way of knowing how painful it is, so to dismiss it, because you can't understand why it is so painful, is uncharitable.
People who don't have SAD will never understand why it's so painful to socialize; but likewise, if you've never been in a painful relationship, you're not in any position to judge how painful they can be, and it's unfair to trivialize the experience of a person in a painful relationship by making statements like, "Well, at least you have a relationship!"
The only thing better than money is more money.