watch keeps asking for my passcode multiple times a day. Why?

Make sure that you have wrist detection turned on for your Apple Watch. You can turn on wrist detection in the Watch app under Passcode settings. If you have wrist detection turned on but your Apple Watch is still asking for your passcode, you might be wearing your Apple Watch too loosely. Make sure that your Apple Watch fits snug, but not too tight, to get the most accurate readings.

Wearing your Apple Watch - Apple Support