I think it varies from person to person, and from time to time for the same person, what makes them happy at work. It also probably depends a lot on the other pressures in their life.
For some people it's more money, for others it's a short commute. For some it's regular praise, for others it's varied and interesting work. For some it's the opportunity to meet people and work with them, for others it's the opportunity to shut themselves away in a room full of computers and only interact with people outside their immediate team by email.
Whilst much derided, more money may bring happiness as it allows you to support your family, finance activities that make you happy or support a charity or cause that is important to you. Money itself may not make you happy but what you can do with it does.
Probably too long (and too true) for them to read.