This is one of several bits of wisdom compiled by Sonja Lyubomirsky in Myths of Happiness.
She applies it to many areas of life, especially to the savoring of experiences and objects.
I think this idea is also helpful in thinking about the relationship between children and happiness. Raising children can make us unhappy at any given moment. Raising children often deals a major blow to our marital happiness for years.
And yet, and yet. Raising children is counted by most parents as the most meaningful thing they do in their lives. On reflection, most parents can come up with many precious moments of delight about their children to savor.
The lesson I take from Lyubomirsky's summation of happiness research on this subject: savor the many small pleasures of children while they are happening. These many small pleasures are they very stuff of a happy life.