Happiness is simple. Good health and relationships, a vibrant spiritual life, sustenance and shelter, feeling useful--those pretty much cover it for me. Simple thankfulness generates much happiness.
It is very difficult to be simple. Especially living in a materialistic society such as ours, we are conditioned that more is better. More stuff complicates life. It's more to take care of, more to break, more to fix, more annoyance, more to eventually discard. "Keeping up with the Joneses," whether it is home size, possessions, kids' activities and performance levels, and/or climbing the corporate ladder, all these make life a complicated effort which does not contribute to happiness. That comes from being grateful for what we have, not the endless quest for more.
Another killer of simplicity is perfectionism. Doing one's best is simple. Having to be perfect in any circumstance adds great complexity. "Perfect" is a constantly moving target, so it is an endless effort to hit the mark. The likelihood of failure is enormous, and the emotional fallout of failure can be very complicated, indeed. Perfectionism and happiness rarely get along.
The go-to example of simplicity in our house is the fact that we don't have an ice maker in our freezer. We make ice the "old-fashioned way," filling ice cube trays with water. We don't have an ice maker to break, leak, and wreck the oak floor beneath the refrigerator. We have plenty of ice and no worries.
Note: in looking for the image below, I discovered that there is now a nearly infinite variety of ice cube trays available. Those aren't even simple any more!