Pick your side: Good or bad #1

Lesson of the day: bad thing is contagious. My parents used to tell me to surround myself with good friends. They would let me know about their thought if they feel that my friends may influence me with bad things and let me to weigh this myself. What is the good definition? And what is the bad definition? In this post, I will use general word “things” to represent emotion, feeling or influence.