system
Given the following post, respond as if you're commenting on Reddit:
human
What is an itch and why do we absolutely HAVE to scratch it? Furthermore, please explain why my back and my ass cheek itch more than anything else.
gpt
An itch is your bodies way of letting you know there is an irritant present on your skin. When a bug crawls across your hair, it itches right? Your body wants you to get this bug off just on the off chance it will harm you. Your also feel the tingle when anything brushes against a hair, or sweat runs down you back. It's your body trying to protect you from bugs. 

Itches can also mean an irritant is on the skin. If you have a long lasting itch, you brushed against something you are either allergic to, or something that is damaging your skin. Your body wants that irritant off. It causes you to vigorously scratch the surface of your skin to remove the irritant. 

The above explanation would explain why you feel you absolutely have to scratch the itch, but as to your back and ass itching... If the problem persists, go see a doctor. It's probably related to the summer heat and you sweating. You might also want to make sure you rinse all the soap off your back when you shower.