Motherhood is a marathon. When someone speaks of it with warm nostalgia, they're sharing a joy created since the particular event ended. Have you seen the faces of marathoners at mile five, when their feet already hurt but they know they're just getting started? Have you seen the delirium of mile fifteen? In the moment precious few of those people look thrilled to be there. After it's over they'll be proud of the accomplishment. The further they are from the day they did it the more fondly they remember the experience. The crazy man screaming obscenities at mile three transforms into singing bluebirds. They focus on the feeling of triumph at the top of Heart Attack Hill, not the pain of going up it.
Motherhood is the same way. No one loves changing the third leaky diaper of the day at the time they're doing it. When they're looking back they don't remember the horror of holding the baby all night because he was feverish and they were terrified of convulsions. They remember holding the baby, a precious life that is comforted beyond words by your mere presence. It's OK not to love the individual moments and it's best to refocus some of the key scenes.