The experience of being a parent. Parenting has its difficult aspects (the sleep deprivation of the baby years; the snarkiness of the teen years) but it has been one of the most profound and soul-satisfying experiences of my life. Getting to teach and shape curious young minds; getting to share my love of nature and books with my kids; watching them say their first words and take their first steps; sharing our family traditions (from picking apples in the fall to dying Easter eggs in the spring); seeing the kind of people they become as they enter their teen years: it has all been tremendously rewarding.
I loved the physical aspect of parenting young children: all the holding and rocking and hugging and nursing. Few things in life are better than kissing a fat little baby tummy or smelling a baby's warm scalp.
I respect the fact that some people (a lot of people on reddit) do not want to have kids. Parenting is not for everybody. But I also feel that people who chose not to be parents are missing out on one of life's richest and most spiritual experiences; and they are missing out on the enormous potential for personal and spiritual growth that the experience of parenting can provide. I don't think one is truly an adult without the experience of parenting.