While this is a worthy goal to have, and ultimately what we should all be shooting for, I don't think the reason for serving is really that important as long as service is getting done. It is not bad to serve others because it makes you feel good inside or because it comes with the promise of root-beer floats. Service is a two-way thing and it is impossible to participate in it without getting something good out of it.
Or maybe I'm just trying to satisfy my own conscience; I usually only serve when there's something in it for me. I willing play the piano in church because it shields me from getting a scary calling (although it didn't keep me from that blasted visiting teaching supervisor calling), I'll go to a service activity if there is a promise of good food, I only go visiting teaching to get my companion off my back, and I only make dinner for my family when I want a real meal to eat. It is really hard to guilt-trip me into doing anything and I rarely do anything unless I want to.
Despite my attitude, lives are still being blessed: my ward has a reliable piano player, someone's leaves are getting raked, maybe someone actually got something out of my visiting teaching visit, and my mom didn't have to worry about cooking for a day. Probably the only one losing blessings here is me because I grumbled through the entire process, but the receiver still got the benefits from my help and the world is a better place.
Service is service, no matter why you do it. Some day I hope to be able to serve without the ulterior motives, that it will just come naturally, but until that time I will serve for all of the benefits I can get. It's hard to get someone as stubborn as me to do something nice for someone else, so avoiding extra work and giving me prizes, treats, and warm fuzzy feelings is a good way to trick me into being a nice person.