Actually, being a negative person can be a good thing. It depends on how you look at it.
I too, have been known as Mr. Negativity, but the fact is, negativity can be very useful.
If people are always thinking positive, looking on the positive side of everything, they may not see or be aware of the dangers, the drawbacks, etc. to their thinking.
Positive people usually only see the positives and ignore the negatives, but the negatives are necessary.
By being negative, you can be more "middle of the road" more easily than someone who just sees the positive.