What is an addiction? What does this mean? How do people become addicted?

I flew into Diego's hometown Mexico 2 hours ago....
The situation seems worse than we thought. According to our information, Rodrigo Diez should be in Mexico city, but we don't know where. We infiltrated his factory 30minutes ago, yet there were only drug addicts in it. We captured an addict for furthur investigation, from him, we found out some information that:

Addiction can be defined as, a mental and physical dependency for something. When a person is addicted, they require a constant dose of the substance, and they develop unwanted symptoms when they stop using these substances. A person may get addicted when a certain substance gives them a good feeling and they try it again to feel it again. However, the initial high of the substance cannot be achieved again, meaning that a person would keep trying the substance to feel the 'highest' point of the substance. This is how most addictions start in people.

Diego suspects that Rodrigo Diez will leave Mexico in a few days, I'll get back to you after analyzing his whereabouts

Dora,