Your outlook upon life, your estimate of yourself, your estimate of your value are largely colored by your environment. Your whole career will be modified, shaped, molded by your surroundings, by the character of the people with whom you come in contact every day.

This quote is saying that the way you think about your life, how you view yourself, and how much you value yourself are strongly influenced by the environment you live in. The people you see and talk to everyday will also have an effect on the path you take in life and your future.