Remember when you were a kid and responsible adults would say things like, "You need to choose your friends wisely."...? The people you spend your time with are the ones who will influence you, your thoughts, and your actions more than you may be aware. If you around positivity and optimism all the time, your brain will take all of that in, and you'll start to be more positive and optimistic, and your default responses and behavior will show it. If you're around people all day, every day, who are negative and cynical, that will rub off on you as well, and your brain will start taking that in, and changing your responses & actions. Think of it this way... if you're around people who swear a lot, you'll find yourself starting to swear more than you may otherwise. If you're around people who eat or drink a lot, you'll start joining them in those actions. So the things that people around you say, and do, and think, and feel, and believe, will start to influence you. Choose your friends wisely. Choose your environments even more carefully. And whenever you can, surround yourself with influences that guide your toward your goals, rather than away.