Even though you know that giving makes people happier, you do not know that telling this to people makes them happier. It may decrease that effect.

Indeed, this is probably my most major concern. For a person versed in cognitive biases, it might seem fairly plausible that our own intuitions about what makes us most happy would be flawed and that the research on the relationship between charitable giving and happiness would be worth taking seriously. But to the average person, suggesting that they engage in charitable giving because it will make them happier is an invitation to assess it as an option on relative to other ways they could spend their money on their own happiness, where charitable giving would probably lose.