“What causes someone to be a pill addict?”

Increased tolerance and physiological/psychological dependence on the drug.

Usually, there is some appreciation for the positive effects of the pills. It can be pleasure, increased energy, pain relief, or relief from anxiety or depression. It becomes an addiction when we can't stand not taking the pills and our brain orients more and more toward seeking, acquiring, and consuming, often at the expense of other responsibilities. Some pills do that, while others don't. If we can't not be depressed without prescribed antidepressants, it's a dependence, but maybe not an addiction. Even when the need has passed, though, coming off some psychotropic medications can be so unpleasant that it seems like addiction and the withdrawal has to be handled very carefully. How that is done depends on the specific pills.
I hope this is helpful.

Take a peek at this link...
https://www.drugabuse.gov/publications/drugfacts/understanding-drug-use-addiction
or
https://theconversation.com/the-real-reason-some-people-become-addicted-to-drugs-81004