To some extent this is true but to a large extent it is not. We can influence others, and we can and should make assumptions; it is how we make sense out of the world. We just need to know the limitations of both our assumptions and our ability to influence others.
Balance. I'm sorry Byron. Though I think I understand the point quite well, and though it may work for you (though I'm not sure why you would be here concerning yourself with the business of others if you totally believe what you say) and I do see some truth in what you say, I think balance is the key. We don't live alone nor just for ourselves, though many self-help gurus would encourages us otherwise. Should a mother not be concerned about her daughter's abusive boyfriend? Should she not say something? Should Annie not sacrifice to care for her Dad who has cancer? Certainly there is value in being able to recognize the difference in whose business you're stressing over, but it is indeed simplistic to think we should just be in our own.