It sure seems to me to be love. Love yourself, love what you are doing, love in anticipation, love others in many ways. I am Christian so this sums it up for me: 1 John 4:8 Whoever does not love does not know God, because God is love. And even in times when I am not feeling all the God in my life. If there is love then things are content.
People, places, things and circumstances come and go. But with love we have all we need to be happy, and more we can be joy filled. If I live my life in love then there is no shame, no guilt, no remorse, no regrets and no loneliness or fear.
We go to 1 Corinthians 13 that most everyone has heard in some form: 4 Love is patient, love is kind. It does not envy, it does not boast, it is not proud. 5 It does not dishonor others, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. 6 Love does not delight in evil but rejoices with the truth. 7 It always protects, always trusts, always hopes, always perseveres.
Living that way will for sure bring contentedness and joy.