The people you spend the most time with shape who you are. They determine what conversations dominate your attention. They affect to which attitudes and behaviors you are regularly exposed. Eventually you start to think like they think and behave like they behave.
The lesson here is to actively construct your social environment. Don’t let it depend on proximity or chance or on how it has always been, but consciously plan which opinions, attitudes and life-philosophies you do and do not allow to be in your life.