Why do you have to shave your head when you become a monk?

First, according to Buddhism, hair represents countless troubles and wrong habits in the world. Cutting off hair is equivalent to removing troubles and wrong habits; Second, cutting off the hair is equivalent to removing the pride and neglect of the world; Get rid of all troubles and practice wholeheartedly. In ancient China, people regarded hair as very important, thinking that it was obtained from parents, so it must be protected and not damaged, otherwise it would be disrespectful to parents. Buddhism demands that these unnecessary family concerns be cut off; The third is to distinguish other sects in India. At that time, India was full of sects. But when people see a bald head, they know it's a Buddhist. Later, shaving became a Buddhist ceremony.

I heard that the monk didn't leave his hair, but he was tired of leaving the Three Realms and didn't want to enter the cycle of life and death.

Buddha and bodhisattva keep their hair, which means that they can't live without nirvana, or they have to go into the sea of life and death to save all beings.