I think it's about our values. For example, if every person you interact with - at some point you say something genuine and encouraging, they will be glad they came across you. It costs nothing to do something nice. But the world is a better place. Like the two strangers in Africa who on separate occasions handed back my wallet full of money and walked off without waiting for thanks, let alone reward. I'm still talking about them.
I usually find the purpose as being moving me forward into the person who I am now. If it felt like they were holding me back, as a few times I thought, it is usually because I was able to move faster with my life.