What is the trigger for Dopamine?

Most dopamine neurons signal a reward prediction error; they are activated by more reward than predicted (positive prediction error), remain at baseline activity for fully expected rewards, and show depressed activity with less reward than predicted (negative prediction).

When everything is familiar, our enthusiasm fades away, and we are looking for new things.

For Dopamine, having is not what matters but getting something new. It's the difference between possessing something versus anticipating it and is designed to keep us alive and make us evolutionarily successful. 

Drugs reroute the dopamine circuit with a more intense stimulation than natural rewards as food or sex.

Different addictions are very similar because it hijacks the mechanism to keep us alive.