I really don't know why I find this man so fascinating. I was not a fan of his policies and they have not by any means improved with age. This is, I think, an interesting question, because I am far from being alone in giving him more attention than, objectively, he seems to deserve. Why is he such a powerful mythos?
His only lasting accomplishments in office, other than the Peace Corps, were the beginning of the Vietnam war and a tax cut that seemed to be (perhaps was) irresponsible at the time (and years later gave the Republicans one of their worst ideas). To these I suppose we must add a series of beautiful images and memorable speeches.
Yes, the fact that he and his wife and his children were physically beautiful is part of the the solution to the mystery of the Kennedy myth. If, as Jay Leno said, politics is show business for ugly people, he had a great (and of course unfair) advantage over the rest of the field.
Being tragically cut down was another advantage, in a way. He became one of those magic people, like Marilyn Monroe and James Dean, who never grew old. They created a few images of glowing beauty while their flaws and shortcomings, which in all human beings are obvious enough, never developed and ripened into something really repulsive.
James K. Polk came into office, announced four goals he meant to accomplish, achieved all of them, and retired after four years. Unlike Polk, Kennedy was not a superlatively skilled politician and did not do much with the three years that fate allotted him. But he did manage to withdraw from the scene before, like Johnson, he had managed to do anything that was clearly awful. So only those images remain -- which is an accomplishment of sorts in itself.
Many die too late, and some die too early. Yet strange sounds the doctrine: "Die at the right time!" Die at the right time: so teaches Zarathustra.