Many persons with bipolar, make RISKY or impulsive choices, have irrational thinking, promiscuity, and a whole slew of other issues. I have a daughter who is bipolar, so well familiar with this topic. There is no alcohol or drugs involved.. but many bipolars will attempt "self medicating" with drugs/ booze. Does not work, but that's not the point. They are at much higher risk. If they happen to walk away from alcohol, they still have the underlying medical issue that can be treated.
Unstable persons are going to be attracted to other unstable people. Just as alcoholics knowingly surround themselves with other alcoholics (as a form of enabling). Its a fast and easy way to avoid facing that there is a huge problem. Two drunks can often be superficially happy with each other. It's when one of them chooses to get sober that the differences become glaring.