Dementors and Lethifolds are a source of negative energy.  They harness the fear of people and use it to their advantage. Much like Boggarts, they can see your greatest fears, and the tragedies of your past.  This is why the only way to combat them is to think of a truly happy memory. 

Only a powerful happy memory can create a Patronus, and it takes the form of an animal with the more concentration you put on the memory.  Concentrate on the memory, let it fill you and lift you up. That feeling of happiness will strengthen your Patronus and counteract the negative energy of the Dementors and Lethifolds.