Releasing dopamine?

In order for dopamine to have any effect, two things need to happen. It needs to be released from a neuron on one side of a synapse, and it needs to be picked up by the neuron on the other side. This happens with receptor proteins on the surface of the cell.

The actual expression of these proteins can be controlled by the cell, though. If you don't have much dopamine released from presynaptic neurons, then the postsynaptic neurons will gradually start producing more and more receptors so that a little bit of dopamine can have a larger effect. If you suddenly release a lot of dopamine, it produces a much stronger response, as the postsynaptic neurons are loaded with receptors.

The opposite problem happens with people who use drugs like cocaine. The drug causes neurons to release large amounts of dopamine. Over time, postsynaptic neurons produce less and less receptors in order to cope with the large amounts of dopamine flooding the synapse. After the drug clears out, there are far fewer receptors present, and normal levels of dopamine can no longer produce a normal response. That's why there's such a strong "low" afterwards, and why increasing amounts of the drug are needed for the same high.

Being low on dopamine makes them particularly interested in the little bits of dopamine that they can get from different activities.

Think of it this way. Someone who is low on food (also known as hungry) will get hooked on doing things that provide food.