Question about MBP and firestick

I use MBP and i have a family account with a few ppl on it, anyway im wondering as we all use MBP on the firestick why it sometimes logs us out of the system and i need to enter the c.ode given to play on firestick. It may go months without logging out and then it may happen twice in a week. Its not a really big issue just curious Anybody know the reason why?