There are countless reasons why you could come to possess someone else's medications. Perhaps your friend felt like they were becoming dependent on a medication prescribed by their doctor, and they asked you to hold on to them to keep them out of the house. Maybe you used a similar medication in the past, and your friend or family member generously agreed to give you the remainder of an unused prescription.
Cohabitating with someone can often lead to conflict that is difficult to resolve. Even the healthiest of relationships and happiest of couples can experience conflict. Everyone handles interpersonal conflict differently, but some people find themselves in loud, even physical arguments with their spouses or partners.