iPhone frequently asks "Trust this computer?" upon connection to Mac [duplicate]
Solution 1:
Most likely lint.
The lightning connector is prone to collecting dust and dirt, which can keep the phone from getting a clean signal from the cable (a faulty or not-to-spec cable can have the same effect). Take a toothpick or anything else that's thin and non-conducting and get that dust bunny out.
Never believe anyone telling you to reboot all devices and reinstall all operating systems in your house.