Why did Iluvatar/Eru give Men mortality?

eru gave each race gifts that were not the same, but were bothe equally "good". Men dream of living forever, but what then? You could wander the Earth, and conquer things and become a king or something(this is sounding a lot like Highlander, isn't it?). The skills of Men are limited, and once you grow to a certain level, you hit a concrete ceiling. I say that if you've done what you want with your life, then you would want to leave it and go to a better place. The elves, on the other hand, are immortal, they do not age, and are graceful throughout their days. One of the main reasons they are allowed into the Undying Lands is because they had to wander the world forever. If you went to every area and every continent, ocean, island, the nwhat would ther be left? Men get pass into a new world (or, as it turns out, the halls of mandos), but the elves are left behind. Allowing the elves into the Undying Lands was partly consolation for having to endure the endless spans of time they must live through, while Men are allowed to leave. So eru blessed both races with their own special gifts, both of them equally "good".