Existentialism, you say?

In class we are discussing the viewpoint of existentialism, which has been very difficult for me. Existentialism seems to cover a whole array of philosophies and it has been hard to grasp. I understand the belief that each person is unique and man is defined by what he does, that part of our discussed I comprehended well. Then we started talking about selfishness and selflessness. That started to make my head hurt. It was argued (or proven, it seemed) by Mullins that there was no such thing as selflessness. That every action, in one way or another, was done to produce a desirable consequence for yourself. I thought about an experience I had during the summer that I thought was selfless. I went to volunteer on maintenance at Kanakuk Kamps for a week after my term as a kamper. I did work projects and set up for parties and events the whole week. I had not anticipated this, but seeing these kids have as much fun that I did the week before felt good, knowing that I had contributed to this. I had not known this feeling would occur, but I knew that I would be serving these kampers. So my motive to stay this extra week wasn't to feel good about my work. I had thought that this act wasn't selfish, but the class discussion had challenged my previous belief. Any thoughts on how/if selflessness can be achieved? Also, I have provided an 'existentialist' joke from one of my favorite shows, Criminal Minds. Ironically (you're welcome Mr. Mullins) I had seen this episode the night before we began our class discussion over existentialism.