No one made the rule. But it exists. Entropy beats everything in the long term.

You can stay interested in life for 100 years. But 500? 5000? A million? It's impossible to know what might happen to the human mind under the weight of that kind of longevity.

Personally, I feel like 80-90 years on earth is more than enough. Then again, that's easy to say now. Ask me again when I'm 80, and you might get a different response.