Usually unhealthy people attract other unhealthy people.
Of course it's not always that simple, sometimes users will hang on to an enabler who seems more outwardly stable than themselves. But if you can't create firm boundaries with people who do all taking and very little giving, then you aren't completely healthy, either.
I've noticed that whomever I'm dating is a mirror for me, like the healthier I've gotten the more stable who I am in involved with is. Of course, the appearance of "captain save-a-ho" (or mistress-save-a-playa?) doesn't mean you're healthy, it means you're exhibiting signs of needing an enabler to clean-up your own messes; usually people aren't genuinely attracted to their "captain-save-a-ho" though. If you're using someone who helps you out, but know you wouldn't be interested otherwise, you're unhealthy yourself.
HOWEVER, I believe that mildly/moderately unhealthy people can get better when surrounded by healthy, loving, strong people. Not in a romantic sense, but like family (or chosen family), a group of friends, a community of people who do yoga together or belong to the same church, that kind of thing.
Usually healthy people are aware of what their issues are, are trying to better themselves, but also aren't convinced of their own perfection or superiority (people who think they're perfect are caught up in their false image of themselves, and are probably unhealthy 1s or 3s).
And yes, 6s are at their core anxious or reactive, and 4s are inherently possessing of strong feelings, so just because you guys don't get along or aren't compatible doesn't mean they are "unhealthy"...just different.