Grief is an inevitable part of living - it's pretty much impossible to avoid losing someone close to us at some point.
Some forms of grieving are less obvious, and we may not even realized we are experiencing an emotional response to a loss.
Examples of this might be:-
One partner has had a serious illness
You have a child with a disability
A partner may have responded to stressful life events in a way that might feel like they are no longer the person we first fell in love with, s/he has changed in subtle and/or profound ways.