Why does it ask for passcode every time I turn it on

I assume you mean turn on the Watch - if you have Passcode enabled, whenever you remove the Watch from your wrist (or power it down), it will require the Passcode when you put it back on (or turn it on). If it is happening a lot, while you are still wearing it, then you are either wearing it too loosely or there is a problem with the wrist detection sensor on your watch. (Unless you have tattoos ......)