When I was growing up, if I was hanging out with kids my mom didn't think were good influences, she would say: "If you go to sleep in the gutter, you'll wake up with rats."
I think about that once in a while, and it reminds me:
It's so important to surround yourself with positive people.
Because the quality of your life is ultimately based on the quality of your relationships. A core ingredient of who you are is who you hang out with.
You can be upbeat yourself, and always look on the bright side of things, but if your personal and work relationships are with people who are cynical and negative, then some of that is always going to rub off on you.
The company you keep is a mirror of your own character.