Help me understand this

I have seen many people here say their spouse has been diagnosed with BPD or Bipolar, or some other form of mental illness. I have also seen some people say, their spouse can hold it together on the outside, as in in front of other family members, friends, or coworkers, but then they show their true "mental illness" self to their spouse. Why is that? Why can they "hold it together" on the outside, but let it all come out on the inside with their spouse? If that is the case, wouldn't whatever mental illness they have, mean they have some kind of control over it? If they can control it in front of others, why can they not with their spouse etc?

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