Happiness is the state of ones mind! In my opinion It has nothing to do with rich or poor. A rich man as well as a poor man, both them can be happy. Yes a stressful life can be an unhappier one. The poverty can be one of the factors making a poor man to have a stressed life and constant pressures to acquire minimum necessities to lead a normal life can definitely put him to stress and this may lead to unhappiness in his life.
The poor people because of malnutrition and unhygienic conditions in which they live, together with the vices they develop because of their social structure, they suffer with a bad health and thus making them prone for many types of diseases. Though these diseases may be different from the diseases what the rich suffer from but they definitely can make poor man's life a miserable one!
The poor man constantly struggles for his existence; he is subjected to various types of restraints because of which he is always under mental pressure and tensions. It is easy to say that a poor may not have worries for tomorrow, but it is not the true. He is always worried about his uncertain future. Though he is born as a poor person, but one shouldn’t forget that he is also a human being and like others he is also having some basic needs apart from just filling up his belly.
To fulfill these needs, which often is not possible for him and thus he is left with frustration at times may get indulged into the crimes, in order to get fast money to satisfy his needs! I personally feel though the happiness is state of mind and though every one can be happy, the chances for a poor man to be happy and to continue in that state for a long time are far less than a rich man.