What about a situation where one party wants to make it work and the other doesn't? In your example, your ankle is under your control - if you want to exercise

What about a situation where one party wants to make it work and the other doesn't? In your example, your ankle is under your control - if you want to exercise it, you can. Your ankle doesn't actively sabotage your body (or if it does spread infection/etc, amputation is often used). A marriage depends on two people working together to build a life together. If one person is not willing to put in that effort, the marriage fails.