Negative energy

I can't understand people who spend their energy on negative thoughts, comments, attitudes as a permanent way of conducting themselves.
Sure, we all have our down moments. Ying and yang; balance is necessary.
But why go through your day finding the negative in everything and everyone?
It drains you, it drains them. Always seeing the "glass half empty" as opposed to "half full" must be a drain on them and surely is for everyone they affect.
A colleague once told me to leave them be, not to take on their monkey. Sometimes that is easier to say than do. But I am learning how to deflect negativity and stay centred and upbeat, most of the time.
For as they say "smile and the whole world smiles with you."
Now, doesn't that feel better?
Have a happy day :)