Answer:
I'm not man's best friend, I'm their enemy, I can mark your end, yet you do not see me, I am very small, but very tough, If you have me, then you have it rough.
A great mysterious place that the bold have been known to journey into.
It is more feared than fear itself, And no one can escape it. It takes no sides and does not judge, it does not know to hold a grudge. It is the most clearest result, Of how life beats you by default.
What types of words are these: madam, civic, eye, level?
What flies forever, Rests never?
You use it between your head and your toes, the more it works the thinner it grows.