From the interview Barack Obama gave to Prince Harry:
Prince Harry: Can I take you back to the 20th of January 2017? You sat in Marine One, the presidential helicopter, flying over Washington. You sat through the inauguration with your game face on. You weren't giving much emotion away, as we saw. What's going through your mind?
President Obama: The first thing that went through my mind was, sitting across from Michelle, how thankful I was that she had been my partner through that whole process ... She is not someone who was naturally inclined to politics, so despite the fact that she was as good of a First Lady as there has ever been, she did this largely in support of my decision to run. And for us to be able to come out of that intact—our marriage is strong, we're still each other's best friends, our daughters turning into amazing young women—the sense that there was a completion and that we had done the work in a way that preserved our integrity and left us whole and that we hadn't fundamentally changed was a satisfying feeling. Now, that was mixed with all of the work that was still undone and the concerns about how the country moves forward. But overall there was a serenity there more than I would have expected.