People have emotions and emotions are contagious. How you conduct yourself and what values you live by define your culture, just as they do the other person's. Businesses have cultures and so do groups, committees, clubs, organizations and any situation where two or more people are together.
The most important influence on you is you. You spend more time listening to your own self-talk than to anyone else, so you need to make sure it's the kind of culture you want to be in. Is it one of your values to put down other people? If not, why would you then do it to yourself?
After that, the people in your work and personal life will reflect what kind of environment you've chosen to be in, and what sort of culture you want to live in.
Since emotions are contagious, the people you're around will influence how you're feeling and sometimes will have more impact than you can withstand. Toxic personalities, for instance, tend to be strong and it's difficult to stand up to negativity for an extended period of time. Somehow it's easier to be cynical, angry and bitter than it is to be cheerful, positive and upbeat, because there are plenty of reasons to be negative if that's what you choose. Pessimists are more often right (Murphy's Law!), but optimists accomplish more (ignore it and carry on!)
Being around negative people can wear you down, exhaust you, and begin to subtly infiltrate your attitudes and feelings.
Surround yourself with the kind of person you are and want to be. Be more of who you are and you'll attract people like yourself. If you aren't optimistic, add it to your mix. It's an emotional competency that can be learned.