Vanity of vanities and struggle of struggles.
The differences that human beings have, in their separate spiritual journeys, are illustrated to us in the Torah almost at its very beginning, with the story of Kayin and Hevel.
It may not have been an innate shortcoming in Kayin. His mother had named him "Kayin" out of the thrill of having borne him. He was her child. And he was her precious acquisition. He belonged to her, and every time she called to him by his name, he heard the importance of acquisition, of ownership.
We are not told why Chava then called her second son "Hevel". But the name speaks for itself. Hevel. Vanity. Emptiness. "Everything is empty."
Now it was Kayin who first conceived the idea of thanking Hash-m for his blessings with a sacrifice. Yet, at the same time, he would not give the best of his produce. After all the best was the most valuable. And Hash-m doesn't need the best , does He?
Hevel, on the other hand, had no problem bringing the best. Who needs the best? All acquisitions are emptiness. If we are grateful to Hash-m, we should show it by giving the best.
Kayin was then zoche to a "mussar shmuz" from the Creator Himself.
Sin, he is told, lies at the door. It is waiting for you to go anywhere, to do anything. But sin desires you - it needs you! Therefore you are its master, and you can control it. Just don't open the door!
We can picture to ourselves the excitement with which Kayin shared this vital information with Hevel [Bereishis 4:8]. "And Kayin said to Hevel his brother......"
Perhaps it was this that Kayin said: "You hear, Hevel? Just be aware when you open the door!" But Hevel didn't respond. He was not concerned with the sin lying at the door. He had no interest in going through that door or any other door. He had no need to conquer or to acquire the world. "Havel havalim; all is emptiness."
Kayin saw Hevel's lack of interest, and suddenly realized that he alone faced a lifetime of struggle. Hevel surely had his own spiritual work to do. But this grinding, day by day, struggle was not his.
When Kayin realized this, he was filled with a rage over the unfairness of it all. He could no longer live with Hevel. "... and it was when they were in the field that Kayin rose against Hevel his brother and he killed him."
Every one of us by nature and by circumstances has his unique struggle. To some of us it seems that others are just blessed by the circumstances of their birth. This makes us sad and discouraged.
Perhaps, it was contemplating this inequality that brought Rebbi to tears [Avoda Zarah 17A]. "Rebbi cried and said 'there is he who gains the next world in many years. And there is he who gains the next world in but one hour'".
Yet, he who succeeds in his struggle is a far greater person than he who struggles less.