Rhetoricals on Follicles

Why does your arm/ leg hair only grow to a certain length and your head hair grows forever?

Why did people find it attractive to part their hair in zig-zags? And did I only not like this style because I couldn't do it to my own hair?

Is it true that your life is more centered when you have a straight hair part?

How does hair know how to grow back when it's shaved off? Especially leg hair. It only grows to a certain length so how does it know when that length has been compromised?

CAPTION: Human hair under 200-times magnification (Wikipedia).