Chassidic thought teaches that before the Flood, G-d sustained the world despite its low spiritual standing, due to His attribute of kindness. There was a limit, however, to how long G-d was willing to sustain a world without merit - hence the Flood.
The waters of the flood were not merely a punishment. They purified the world, making physicality in general more refined, and spiritually attuned. [see The Great Flood]
Consequently, in the post flood era, people were more predisposed to repentance. This ensured that G-d would always sustain the world - not despite of, but - because of its spiritual standing. For, even if man would become corrupt, people would inevitably repent, ensuring that the world itself would have sufficient merits for its continued existence.
With this in mind, we can explain the following details:
- Noach was unaware of the above, so he was scared to repopulate the world, fearing it would be destroyed again. Therefore, G-d had to re-command him to ''be fruitful and multiply''. [Noach 9:1]
- The inner reason why Noach's generation failed to repent was because, before the Flood, the world was spiritually insensitive.
- Meat is an extremely coarse food that can lead a person to excessive physicality. Thus, it was only permitted to the spiritually-attuned post-Flood generation. [Noach 9:3]
- Before the Flood, people had extremely long lifespans because the world was sustained by G-d's kindness which was bestowed disproportionately to people's merits.
- Before the Flood, physicality was more coarse. This was true in a literal sense, to the extent that the clouds were too thick to refract light, so a rainbow never appeared. After the Flood, physicality become more refined, so the clouds began to refract light. Thus, the rainbow was not only a ''sign of G-d's promise not to destroy the world, it was also a physical consequence of the refinement of the world that ensured its permanent existence.