Playing video games and surfing the internet results in dopamine being released by the presynaptic neuron into the synapse where it's received by the receptors on the surface of the postsynaptic neuron.
This is the part of the brain that transmits pleasure. It is the same pathway that is present when someone becomes intoxicated with substances.
https://www.youtube.com/watch?v=7VUlKP4LDyQ (Check out minutes 2.18 – 5.00 for a visual illustration)
https://www.simplypsychology.org/brain-reward-system.html (Suggested overview)
Why is dopamine a problem?
Dopamine is not a problem when it rewards a healthy behavior. But when the pleasurable activity is done excessively for hours on end, it can become worrisome.
When a brain is used excessively, it gets tired. It needs to save energy. So it does something called downregulation. Downregulation erases some of the dopamine receptors on the surface of the postsynaptic cell. The longer an activity continues, the more downregulation occurs. The result? There are now fewer receptors to catch the dopamine. The user has created a tolerance. He or she now requires MORE dopamine to acquire the same amount of pleasure. This is a reason why people who play video games for hours and hours may not experience as much pleasure in their everyday activities.
Furthermore, since the child now has fewer receptors for dopamine, he/she is actively absorbing less dopamine. This is called withdrawal. During this time you will likely see the child going through moodiness, irritability and anxiety.