Don't be so negative!
Usually in my experience they are saying this because
- they have failed to understand the situation properly
- they are trying to be motivational to other people (or sheeple) who are listening,
- they think it makes them sound positive
- and they think that is always a good thing.
Each and every time I hear 'Don't be so negative' I wonder whether the speaker has realised the irony of their outburst.
After all - just tell me how can you tell someone not to be negative without being negative yourself?
Small note: Speaking as a (sometimes negative) skeptical pedant (the last two of which I admit and celebrate) I feel uncomfortable with the lack of a question mark in that first paragraph. Even so - I can't decide where to put it. So here it is . . . ? I feel a need for the Spanish technique of using an upside down question mark symbol to start a question, rather like quotation marks.