What is ____?

It is like a snake, with a beginning, and an end.
It's head is as defenseless as a newborn pup.
It's neck, a flower, delicate, and fragile, but no matter how much you step on it, it still stands.
It's body, a stone, remains the same throughout the years; but when you look closely, it is constantly changing.
It's tail is long, short, large, and small; it is the hardest part to see of them all.
It can go by in a flash; but when it stands still, motionless, it's as if it wasn't there at all.
At times, it's skin is smooth; at times, it's skin is scaly, and gets twisted.
When it smiles, it brings happiness and joy.
But when it cries, it brings suffering and pain.
You may wish for it to flee, but when it's gone, you will weep.