Simplicity is purity. It is facing the true nature of things and embracing it instead of ducking and weaving and dodging, instead of filling a hole inside you with chaotic activity or an overabundance of stuff.
One of my key realizations about happiness, and a point oddly under-emphasized by positive psychologists, given its emphasis in popular culture, is that outer order contributes to inner calm. More than it should.
It's not always easy or fun to change things, but when you do it frees you. These days what I want is smaller and less. I want more time to do what I enjoy and to be with those I care about. That's what makes me happy.