Why do we often end up with partners who have similar emotional health levels as us?

Have you ever been blamed for problems in a friendship you didn't know existed? How did you handle it?
What do I do? I think my boyfriend is cheating but he swears that he isn't. There are multiple red flags he still swears.
Have you ever been caught in a funny predicament because you tried to outsmart someone in authority? How did it all play out?