One of the most well studied genetic risk factors for addiction vulnerability is the DRD2 gene, which encodes the dopamine receptor D_2. People who are particularly vulnerable to addiction commonly have a deficiency here, meaning getting bored very easily and feeling under stimulated/rewarded by typical things is in part what makes riskier sources of pleasure (i.e. drugs) so appealing.
Disclaimer: Not a medical professional, just like reading about addiction and genetics.
A couple of the people I knew who ultimately ended up in rehab would describe themselves as not having addictive personalities. One even bragged about how boring drugs had become to himself.
This is the hubris that opened the window to their addiction. They thought it was a thing that happened to other people, so it was safe to dabble. Then dabbling became a little more frequent, then they decided the drugs were actually helping them and therefore it was logical to do more, then they took little breaks to prove that they could stop but came right back to the drugs as soon as their life got a little hard again.