You meet a broken person, just like you once were. This person shows you how wonderful life can be when you're around them. You grow fond of their company and unknowingly sew your care, respect, and empathy for them--until it becomes a thick, in-destroyable piece of handmade clothing. To you, "fixing" them is something you wish to do for either two reasons: 1) personal satisfaction from the fact that you were able to bring light into someone's life or 2) solely want them to be better off.
When you try to fix someone broken, it's even worse. Here, you are dealing with countless puzzle pieces...from different puzzle boards. Their reasons are scattered, their feelings are unexplainable, and their struggles are hidden. Broken people come in different shapes, sizes, and backgrounds.
They may be "perfect" in every aspect. They may have given you affection or good company in a way no one else was able to. But that does not mean you owe them anything. If it messes with your own self, then it's a major red flag.
When you try to fix others, you place yourself on the bottom of the meter. You see the world in pain. You attempt to read people and their eyes--but it is not your job to do that. You might feel great about doing that because it reminds you that you are selfless. But, since when did being selfish become a bad thing? Why do you not allow yourself to think about your wants and needs for once?
You lose your patience, mind, confidence, and trust. You lose the key components to safe and good mental health.
It is important to grow with another, but it doesn't mean you should stick with someone when they are changing the way you feel about yourself on a daily basis.