Why is caffeine not good for you?

Caffeine can actually be very good for you.

However, like all chemicals which act upon the nervous system, constant or high usage can result in the body becoming dependent on it. Because of the way the CNS works, stopping such usgae results in the effects it counteracts being much more pronounced for some time (probably due to the increased amount of chemicals released to counteract the caffeine).

Like all drugs and chemicals, the things itself is neither good or bad - it is how you use it that matters.