Before I begin, I think part of the confusion here is that there are two brands of selfishness. One is a psychological illness, where a person is unable to give, share, or love. The other applies to people who work to fulfill their dreams and ambitions, and who create the life of their dreams.
To gather information on this topic, I went on a field trip to a doctor's office, a drug store, and a grocery store (just because the drug store didn't have the right kind of diapers for Charlie).
I wanted to see why people do things. There was a receptionist at the doctors' office who had a big smile on her face. I wasn't sure if it was her hot date last night, or if she was high on some cool drug, or if she was just a smiler. Then I saw the nurse to give me a shot. She was trying to be helpful, giving me screwy information that made no sense in my situation. I tried to be nice about it. Both of these people are paid for their services and wouldn't do their jobs if it wasn't for the pay. They did their jobs relatively well ... well enough that they were still there after an extended period of time. Were they essentially benevolent beings, or were they just doing what they were asked to do? You tell me.
Then at CVS (the drug store), a nice clerk asked me if she could help. I knew that if I just said "diapers" she'd think they were for me. So I asked for "baby diapers" (though I don't think of anyone but a complete new born as a baby) and she told me what aisle I should go to. Again, a helpful person, paid to do a service. Would she stand there without the pay? I doubt it.
I pulled my car into the parking lot of the grocery store, and noticed a woman in the car next to me loading her groceries into her trunk. It looked like she had forgotten two large cans at the bottom of the basket so I mentioned that to her. She said that she had not forgotten them. Then I asked if she'd like me to take her two carts. She looked tired and I thought that would be a nice gesture. It actually gave me a lot of pleasure to do this for her, especially after she smiled and seemed appreciative. I didn't debate with myself about whether I should do this or not. It seemed like doing this would make the world a better place, and would make her day a little better. It gave me a lot of pleasure to contribute this positive energy to her world.
Driving home, I started thinking about how and why my wife puts hand cream on her hands. Is this an altruistic act? Surely it would be if she just did this for me. But actually I think she treasures her hands and treats them with respect. I'd say it was primarily a selfish act.
As I drove home, I saw a golf course, streets, stores, telephone and electrical wire, cars and busses. All of this was made by people who have no particular attachment to each other ... yet it is these “selfish” acts that make the world go around. Sure we have the good Samaritans, but generally most of what we do, and what others do, is quid pro quo. I do this for you and then you do something for me.
The primary goal for the Buddha (I think of Buddha as a view rather than a man) is to relieve suffering (sometimes translated as anguish). What better way to do this than to develop and practice skills that make the world a better place? Why do we do it? Usually because that's our job. That supports the people we love and ourselves. That contributes to our feelings of self-worth. Do we relieve suffering? Absolutely. Is this what makes the world go around? Most assuredly.
P.S. Please watch Milton Friedman's piece on ”I the Pencil.” He describes how many people, with nothing in common, with neither love towards or affinity with each other, produce a common good. He once told a mom, "you are primarily concerned with helping your family. I'm concerned with helping the world." Is this far from Buddhism?