Pair Apple Watch again without erasing
I was experiencing this problem when i had my phone on my desk and I would walk away from it. When I came back I found that the watch would refuse to re-pair with my phone again.

I found a quick way to force the watch to re-pair with my phone:


Flick up on the watch face to get to the "Glances" menu
Tap the airplane button to turn on Airplane mode
Tap the airplane button again to turn off airplane mode
Within a few seconds the watch will re-pair with the phone