Why do communities make their own traditions?

Every town, every neighborhood, every group of friends seems to have its own special way of doing things. One place parades a giant papier-mâché dragon through the streets every spring. Another holds a pie-baking contest on the first Saturday of summer. A third lights lanterns and floats them down the river when the leaves turn gold. Why do communities invent these traditions? Why not just borrow someone else's, or skip them entirely?
A tradition starts when something happens once and people decide it felt good. Maybe fifty years ago, someone's grandmother baked an apple pie for the town picnic. Everyone loved it. The next year, two neighbors brought pies. Then five. Then twenty. No one wrote a rule. It just became "what we do here."
