I don't have a single reason but numerous:
- Sexy all around device with amazing details and feels extremely well built
- iOS
- Apple's ecosystem (Seamlessly works well with all my Macs, iPad, iCloud)
- Simplicity and ease of use
- Appstore with a lot of high-quality and premium apps
- UX/UI Is the best of any smartphone out there
- Gorgeous Screen
- Amazing Cameras
- Overall design
- Airplay
This for me too. Plus wealth of accessories and service stores.