There is enough converging evidence that we do influence each other to basically say, look, let's assume that there is some prediction going on and then try to unpack it. I can see a lot of ways in which yes, friends, especially in your teens twenties, unless when you're at the advanced stage that you and i are, could have a deep influence on how your life actually turns out. So i am coming down a little bit on the side of yes here, with reservations.
Also: which professions have the happiest people?