You're messing with the reward pathway in you're brain. When you take adderall it sets off a mechanism that begins to release dopamine and norepinephrine from neural synapses in the prefrontal cortex, basil ganglia, and other parts of the brain. Taking it in doses greater than prescribed increases levels of dopamine firing in between the synapse to the point of intense euphoria. This results in ecxitotoxicity, meaning the neurons die off due to it's inability to break down certain chemicals in the process.
So in addition to coming down feeling lower than you felt before taking amphetamine, you gradually alter your brain's capacity to feel pleasure from everyday activities like socializing, eating, sex, etc. If this begins to become habitual you eventually get to a point where you take the drug just to get back to baseline (aka feel normal), resulting in even greater degeneration of key functions in the brain.. Rinse, wash, and repeat and you begin to understand why stimulant abuse is considered one of the great global scourges of humanity.
Taking stimulants is like using a credit card. Yeah you may be coasting along feeling invincible while using them, but the brain and body ALWAYS end's up paying for it somewhere down the road.
Don't rack up too much debt.