Not to discourage more answers, but you might want to see the relevant section from the FAQ:

• When/why did people start cutting their hair?

• When was the first haircut?

• How did people take care of their hair throughout history?

Kind of confusing question, need to be more specific. Hair trends have been in a cyclical movement throughout history. From East to West, and from age to age, hair/beard trends would change. Men would be clean shaven (head and beard) in Acient Egypt, and then was adopted by Greece/Roman for style, and to look "modern". Then long hair made a come back. If you look into the past, you'll find being clean shaven, full beard, long hair, or short hair isn't as modern or trendy as societies claim. Nothing original.

Also the why, doesn't originate in one place or time. Every society/individual, in the past and present have their own reasons.

I'm Muslim, and in Islam, men have a hair length limit(to the shoulders), because long hair is reserved for women. This is our "why", however, other societies, will have different reasons for keeping hair short, or long.