How do couples remain bound through extreme struggles?

I read about an extremely good looking married couple that want to climb Mount Everest together to raise money for cancer research and my dads friends are a married man and woman who spent an entire day climbing the Grand Canyon together in their 50s to 70s. I don’t know how somebody can be that strong cause when my muscles are exhausted enough, not even a gun and a death threat can motivate me to continue with my work out, I will stop my work out, even if it means death from hunger or dehydration. Also a couple that good looking does not need to climb Mount Everest to get attention and respect.

How do couples remain bound through extreme struggles?
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