Dementors

J.K. Rowling introduced an interesting concept. Harry Potter has creatures called dementors. They feed by consuming the happiness in the air around them. When dementors are around, people start feeling depressed, and relive their worst memories. Those kept in the company of a dementor for too long are often driven insane

I have actually seen people like that in real life. They suck the happiness out of everyone around them. They create negative energies everywhere they go, and feel a sense of satisfaction by making people sad. That satisfaction is driven by a hatred towards everyone. People actually say things like "he's driving me insane" about them.

I think hope and spirits follow the pattern of heat. They flow from higher potential to lower potential. If we are around an energetic and high spirited person, we start feeling some of that energy. Vice versa for a depressed person. The dementors keep their own potential so low that all optimism around starts flowing into them. It takes a strong person not to be affected by a dementor.