The Elves have it easy. They're immortal, first of all. This doesn't mean they can't be killed; they can. But if they're not murdered they'll live forever. And, perhaps more importantly, if they are killed they know what happens to them: their souls go to the Halls of Mandos, where they wait. Kind of like the ghosts in Pac-man.
Furthermore, the angels (the Valar) love them and will do just about anything for them. The world the Elves are created into is innocent, and as soon as the Valar hear the Elves have popped up they rush to meet them and welcome them with open arms into Eden. Once there, as we've seen, the Elves figure out a way to screw things up - by making Things - but it's important to note that, while this dip into materialism ultimately dooms the Elves, and can thus be likened unto Original Sin, it's not characterized as a Transgression. The Valar never say to the Elves, hey, don't make a Silmaril. And in fact when the Elves make Silmarils, the Valar think they're pretty cool.
Men don't have it so easy. The world they're created into is one already under the sway of Morgoth, and no angels come running to see how they're doing, much less Elves.
Furthermore, and this is the big difference, Men die. They die no matter what: slain, sure, but also of old age. And when they die, they really die. The Elves don't know what happens to them. The Valar don't know what happens to them.
Now the Valar and the Elves insist that this is a gift the creator gave Men. "Death is their fate, the gift of Ilúvatar, which as Time wears even the Powers shall envy" (p. 42). But try telling Men that: Hey, you know, you're going to die, and we've got no clue where you're going. Us? We're going back to Eden, but you? Tough titty. But don't get all sore about it: it's a gift, don't you know.
This gift, we're told, is part and parcel with Men's nature, which is to "seek beyond the world and...find no rest therein" and thus to escape fate, which rules the Elves - Men are free, in other words, in a way that Elves are not. Free, but given precious little guidance, it seems, save what they can beg from the Elves, if they're content to serve the Elves as stalwart followers - and let's remember that the Elves call them "Atani...the Second People; but they called them also Hildor, the Followers, and many other names: Apanónar, the After-born, Engwar, the Sickly, and Fírimar, the Mortals; and they named them the Usurpers, the Strangers, and the Inscrutable, the Self-cursed, the Heavy-handed, the Night-fearers, the Children of the Sun" (p. 103).
Men, we're told, are easily seduced by Morgoth and then Sauron, who play upon Men's fears. Fear of, it seems, death above all. And in the story of Numenor, we get an up-close view of how this works. And it works ironically.
So the Quenta Silmarillion tells us how the Elves screwed up. They mostly evacuate Middle-Earth in the wake of their disastrous experiment in living outside Eden. Then the Valar feel guilty for all the stalwart Men who died fighting at the Elves' side - Men didn't make the Silmarils, after all - and they give these faithful Men an island, Westernesse, or Numenor.
This is Men's Eden, a land of peace and plenty. But unlike the Elves in Valinor, Men don't dwell with the Angels. Numenor is a formidable distance from Valinor. And here's where Tolkien uses the apple motif from Genesis. Eden/Valinor is on earth: if Men sail west from Westernesse long enough they'll reach it. In fact they can see it from their highest tower on a clear day. But the Valar set a ban on them, that they're to never sail west out of sight of their own shores, and above all they're never supposed to sail to Valinor.
Here's Eden: but you can't go there. Now this is, like the apple, the kind of rule that's just begging to be broken. And of course Sauron shows up and starts playing on Men's fears, pointing out that the Valar and Elves in Valinor are immortal, and why aren' t Men? Why do they forbid Men to go to Valinor and share in that immortality? And so of course eventually the Numenoreans do, and that's all she wrote: the Valar sink Numenor, and the remnant of Men, the only ones who didn't want to eat the apple (so to speak) flee to Middle-Earth.
So that's the condition of Men in Tolkien. They're subject to Death. Nobody seems to know why, or in fact just what Death is to Men. They're told, once in a while, to take it on faith that Death is a gift, but just what kind of gift nobody's sure - and it's not like the Elves are in any hurry to find out for them. Men are afraid of Death, and it makes them weak and persuadable. Elves look down on them for this, as does the narrator of every Tolkien story I've read (and I haven't plunged into the History of Middle-Earth yet). And yet nobody seems to offer Men much help in overcoming their weakness, or understanding it.
It seems like a pretty sorry lot indeed, and from the point of view of The Silmarillion, it is, since The Silmarillion is a pretty quendicentric book. And yet I suspect that in the final analysis, all the Elven-immortality stuff, all the Valar/Valinor/Iluvatar stuff, is there precisely to create this gloomy outlook for men. I suppose I think it's an anthropocentric literature, after all. In the end what Tolkien's after most of all is that dark Norse worldview, where Men are doomed, end of story. Only against that kind of grim metaphysical backdrop can we see true heroism. Think of how many times in LOTR Aragorn or another human says something like "I keep no hope for myself." That fatalism is what comes through most strongly in Tolkien. To me, at least.
It's very hard for the modern mind to wrap itself around, I think. Men get a raw deal, but they're not allowed to point that out. There's no heroic rebellion in Tolkien, no hint that Men are at all justified in fearing death, desiring immortality, or in any way being discontented with their lot. Tolkien was in many ways a Romantic, I believe, but there's no trace in him of the Romantic identification with Satan.