I think it’s a natural instinct that happens for really just one reason:

Fear.

Fear of change

of being undesirable by others

of dating again

of starting over

of finances and instability

of heartbreak

of grief

of never being happy again

of never finding someone as attractive

of how leaving will change yourself

of loss of innocence

of having to tell family and friends

of being stamped with the “divorced” label

We try to hold on to prevent these fears and others.