Some brain cells make dopamine to reinforce behaviors. Dopamine slots into receptor, you get that feeling of satisfaction. A protein in the brain cell wall vacuums up excess dopamine so it doesn't just flood the brain forever, constantly triggering the receptors.
ADHD brain cell dopamine vacuums are just always on max speed all the time.
Methylphenidate slows the vacuums down so the dopamine doesn't get sucked right back down, meaning it actually has a chance to reach the receptors and do its job.

It's not quite that our brains crave dopamine, it's that our brains are too good at tidying it up, kinda like someone following around behind you as you set the table, putting all the clean dinnerware straight into the dishwasher unused.