something that i can't deny

Something that I can't deny is that some relationships can be useless or can be seen as well as be used as a life lesson and when I say relationships I mean dating, friendship, etc. This is my honest reasoning on something that I cannot deny. The only reason that I look at relationships this way is because I see different situations in a lot of relationships. To me friendship is the most important type of relationship apart from family relationships of course. Some of the things that I have seen is violence, gaslighting, abuse, misogyny, and psychotic type of behaviors the question is what can cause these in all these types of relationships.