Negativity and positivity have been shown to be contagious. Consider

the people with whom you’re spending time with. Have you noticed that when some are in a bad mood, they can bring down almost everyone in a room?
A positive person has the opposite effect on others. Being around positive people has been shown to improve self-esteem and increase your chances of reaching your goals.
Surround yourself with people who will lift you up and help you see the bright side of life.