“It is inevitable if you enter into relations with people on a regular basis…that you will grow to be like them. Place an extinguished piece of coal next to a live one, and either it will cause the other one to die out, or the live one will make the other reignite. … If you consort with someone covered in dirt you can hardly avoid getting a little grimy yourself.” — Epictetus

Or more simply:

“You are the average of the five people you spend the most time with.” — Jim Rohn