I don't grasp the new generation concept of needing the latest gadgets, paying huge premiums ($1000 for a phone and $300 per month for plans is absolutely bananas), standing in lines, etc.
Considering I detest giving money to technology companies, given their relatively corrupt evil practices, and considering constant upgrades are not environmentally friendly (for all the treehuggers out there to take note), and given the full day hassle of the transition, I upgrade only when absolutely necessary when I've squeezed the life out of my current mobile phone.
That's generally about every 3-5 years. Then I'll buy a $100 phone and start the process anew.
I got my first cellular around 1999 or so. That got smashed by accident after a few years. Since then I think I've had four or five. The reasons are generally that the phone simply no longer works reliably. My favorite was probably my Blackberry. I really liked it. High quality, durable, I liked the roller ball and the physical buttons, and other features like the ringer and vibration that no other phones seem to get right.