This does make sense, our brains are hard-coded to perform acts of altruism and without doing it then we're not satisfying a need of our body. I suppose this depends largely on the person, some people get cranky with low blood sugar so I wonder if some people get cranky when they don't do nice things.

Another interesting thing I've learnt on happiness is that you can make yourself happy (especially when you're sad) by simply smiling. I've heard numerous explanations as to why (the best is that performing the physical action causes the nerves to activate that cause you to smile when you are happy and in turn actually begin to make you happy) but I just know that it works.