Research that tells you something you already knew: Givers are happier

Sure they are. The research is most interesting, even though the conclusion is not very surprising. In "Why Givinf Makes You Happy", Arthur Brooks doesn't really explain why giving makes you happy but he provides some very interesting information:

The happiness difference between givers and non-givers is not due to differences in their personal characteristics, such as income or religion. Imagine two people who are identical in terms of income and faith — as well as age, education, politics, sex, and family circumstances — but one donates money and volunteers, while the other does not. The giver will be, on average, 11 percentage points more likely to be very happy than the non-giver.

Do people give because they are happy or are they happy because they give? Actually, it is more likely a feedback loop - it is mutually reinforcing if you keep it up.

Not only that but if you expect giving to make you happy, it is more likely to do so. There is some interesting neuroscience stuff here too:

For example, people who give often report feelings of euphoria, which psychologists have referred to as the "Helper's High." They believe that charitable activity induces endorphins that produce a very mild version of the sensations people get from drugs like morphine and heroin.

Charity also lowers the stress hormones that cause unhappiness.

Just don't get sucked into the belief that it is some purely chemical process. For example, one reason that charity will reduce the stress hormones that cause unhappiness is that it causes our minds to stop focusing on our own (unhappy?) feelings and start focusing on the happy feelings of the person who is receiving a benefit. Just not thinking about our own troubles for a bit is a benefit to us, and results in greater self-reported happiness.