Rashi: This is why I told you to turn to me. How can I be silent from screaming over my breakage? You know that there is a fixed time how long man will live!

Malbim: He refutes Elifaz' claim that Hashem afflicts in order to atone for a light sin, and to exempt from a greater punishment. This is wrong. Man was created to perfect himself. (a) Perhaps this is via unceasing Avodah all his years. His allotted lifespan limits his perfection. His lifespan was allotted in order to enable him to reach his perfection. This is like a worker who must serve his master all his life from dawn to Tzeis ha'Kochavim, whether or not he will finish the job. (b) Refer to 7:1:2:2.

Rashi: He was hired for a year, and he knows that his contract of servitude will end. So man knows that he will live the years allotted to him.

Malbim: Or, perhaps perfection depends on a limited task. E.g. if man will fulfill 613 Mitzvos, he perfected himself, whether this was over many years or in a short time. However, if he did not finish it, even if he lived many years and served Hashem in all of them, he did not reach his perfection.