Negativity is contagious
If you disagree then I have some property to sell you in the middle of the Arctic Circle! Seriously, what people think of themselves, their circumstances, or you can affect what you think about yourself, your circumstances, or your thoughts of others. It's a law of human nature.
If you surround yourself with sweet you smell sweet. If you surround yourself with sour you smell sour. But here's the difference between negative and positive statements; the negative statements cause the greatest shifts in our thinking and behavior. Not only from good to bad, but bad to worse. Another law of human nature. This has all the trappings of group think.
You see, the opinions of others exert strong influence on your attitudes particularly when their opinions are negative. This impacts your perceptions. When your perceptions are faulty or negative you lose out on life. On the future you are striving to move toward.