In Parshas Noach, we find several striking changes after the mabul. First of all, lifespans are shorter. Secondly, Noach and his family are given permission to eat meat, in addition to plants and grass (9:3). Why does life expectancy change so dramatically after the mabul? And why is man suddenly permitted to eat meat?
After the mabul, Hashem promises Noach that He will never again bring a flood to destroy the world (9:11). What’s more, He offers the rainbow as a sign of this pledge (9:12-17). Why does Hashem need any reminder of His covenant with man? Shouldn't His promise be sufficient?
Earlier in the parsha, Hashem tells Noach that from now on, the seasons will always follow their natural order. “All the days of the earth, planting and harvest, cold and heat, summer and winter, day and night, shall not cease” (8:22.) Immediately afterwards, Hashem grants permission for Noach and his family to eat meat, and then He mentions the rainbow. At first glance, the statement about the seasons and the section about the rainbow should have been placed one next to the other since they both discuss Hashem's promise to never flood the world again (see Rashi there). Why is the permission to eat meat placed between them?
The Sforno explains that when Hashem said that after the mabul the natural order will never change, He did not mean to declare merely that the seasons will never cease, but rather He was introducing the whole concept of seasons. Before the mabul, a year was actually not divided into different seasons because the earth was not tilted on its axis. It felt like spring all the time. The climate was ideal for plant life and for people. Fruits were qualitatively better and more plentiful. Human beings were healthier. Their constitution was more wholesome. But after the mabul, Hashem tilted the earth on its axis which brought about the different seasons. As a result, food was no longer as abundant and life expectancy diminished.
Perhaps that is why Noach and his family were given permission to eat meat after the mabul. It was not a reward for saving the animals during the flood as the Ramban (Bereishis 1:29) suggests. Rather, as the Malbim (9:3) puts it, since human beings became weaker after the mabul, they needed meat to strengthen themselves, to give them extra nutrition.
This can also help explain why the permission to eat meat is mentioned immediately after Hashem’s statement about the seasons because the two are directly related to each other. Once the seasons began, there was a need for human beings to eat meat, and that is why Hashem granted them permission to do so.
But the question remains: why was it necessary to create seasons in the first place specifically after the mabul? One possibility is that after the mabul, Hashem wanted people to feel a greater sense of urgency. He did not want them to think that they could waste hundreds of years of life involved in all kinds of trivial pursuits and still have plenty of time left to devote to more meaningful endeavors. Perhaps that is why Hashem created seasons and shortened the lifespan of a human being, so that man would appreciate that his time is limited and he would make the most of every moment.
Chazal comment (Brachos 5a) that a person should always try to overpower his yetzer hara with his yetzer tov. But if he is not successful, then he should study Torah, and hopefully that will focus him. If he still cannot subdue his yetzer hara, he should recite Shema, which contains an acceptance of divine authority, and the concept of reward and punishment. But if even that does not calm his yetzer hara, then he should remind himself of the day of death. When all else fails, remembering that life is not forever can return a person to the proper path.
This perhaps is the deeper meaning behind the rainbow. It is not a reminder for Hashem. Rather, it is a reminder for all of mankind to stay focused on what is truly important and not become distracted by the lures of the material world. Chazal observe (see Rashi 9:12) that during generations of great tzaddikim like Chizkiyahu and Reb Shimon bar Yochai, no rainbow ever appeared. In those times, it simply was not necessary. When people naturally have the proper values and perspective on life, there is no need to remind them to live for a higher purpose.