It's a nice simplistic theory I don't agree with. There are so many other factors beyond ouselves and our thoughts, that influence our lives. How we treat one another has a big effect and is most important. And people really can ruin the lives of other people, no matter how positive those people may be.
As to people ruining the lives of other people, your life is ruined only when you accept the notion that it is. You choose how to view your life. Others control you only to the degree that you allow them.
Stuff happens, that's a given in life. Other people make bad decisions that sometimes affect us. The terrorists that blew up the Trade Center made some bad decisions that affected millions of people. After 9/11 we, as a nation and as individuals, had the choice to either curl up in a ball and suck our thumbs whilst hiding under the table in fear or we could rebuild and do something about the situation. President Bush chose to fight back and not let the bad guys determine how we will live. We as individuals, have the same choice. Whatever happens in our lives, no matter how awful it may be, we always have the choice not to let it determine our happiness.