After some time, their sub-conscious mind will start believing what they are saying and start acting accordingly. Soon they will have reasons to be unhappy for real. Do not get into this trap. See the positive rather than the negative in you. Optimism and a positive attitude promote good health.
Research shows that the optimistic persons have healthier immune system.
Pessimistic expectations breed negative experiences. No matter how bad things might appear, you can see a silver lining. There is someone who has worse problems. Misery loves companions. Reach out to such people.