Let me explain something.
Back then, everyone was super religious. It was still the beginning of the world, you know. And everyone wanted to die and go to heaven. Screw real life, what's that compared to heaven?
But, the problem was back then you couldn't be too sure what path to follow. Also, since you lived so long for all you could know you could have been doing 900 years of destruction.
So when Noah heard God's voice, he was as happy as hell. All he had to do was listen to that voice, and he, and almost everyone in his family would be guaranteed heaven.
So yeah, he was more than happy to do 900 years of service, compared to eternal bliss.