When I was 8 and my parents would fight(years of terrible screaming, no idea how the cops were never called) my little sister would hide downstairs in a closet or in the corner behind the couch or a number of other spots and just bawl her eyes out. I'd always go find her and cover her ears or sing to her or make her play some kind of game to distract her. It was weird at that age fully understanding how harmful my parent's behaviour was and seeing that they didn't understand. That's when I started looking out for her because I realized they wouldn't always think of her.
There was also when my mom left my dad. At the exact same time my older sister decides to up and move 900km away and never talk to any of us again. They had been close all her life and she just decides to change her phone number, delete facebook, and move. He was in a daze, having trouble functioning, he'd go for drives for hours, I'm pretty sure he'd drink every day. This was a year or so before I moved out of the house. When I was there I'd stop it by putting my arm around him and walking with him instead of letting him go drive. He told me that all the times I walked with him got him through that time. It was a rough few months.
Then there was the lady I was supposed to marry. I helped her through months of depression. I was 19 and she was so down that she had trouble leaving the house, no job, no doing things with friends. I was working at a series of retail jobs trying to support the both of us, her car payments, our rent, her old line of credit and credit cards. It put me 15k in debt before the relationship failed.
It all made me realize that no matter how strong I was everyone on the planet is badly damaged in some way, and not all of them have the ability to manage their broken pieces as well as I do mine. I can coach people, lead them along in the right direction, give them tools to fix what's wrong, but if they wouldn't take the help I couldn't take responsibility for it. I had to help the people I love but not put myself in a position where my life will be ruined by other people's problems.