Yes, the promotion of the selfish gene is responsible for the human behaviors, because people and animals try to do things that make them happy. Most people honestly only want to do the things that they think will maximize happiness, even if that means delaying gratification to get something they want the most.
Yes, I believe a selfish gene makes it so that humans are, when it comes down to it, very selfish. I just feel as if we have so much trouble connecting with others and putting our own interests aside for the common goal. I especially feel this way about Republicans.
There are social barriers that doesn't allow you to go out and get what you want. When you do jump these hurdles it can effect the people that held those hurdles and can interfere with you. People have to act out differently to jump these hurdles which can actually benefit others or worsen the condition for others.
Human behaviors cannot be reduced down to genetics and the behaviors of genes. Behavior is something that arises from more complex interactions, which include social dynamics and the environment. People are predisposed to behave certain ways based on genetics, but environment plays an important role in shaping behavior. Epigenetics is the study of the way environment actually causes the expression of certain genetic traits, so their is an understanding in the scientific community that genes alone cannot explain behavior.
I do not believe the promotion of the selfish gene is responsible for all human behaviors. Genes are a relatively new finding to humans. Humans have been acting in similar ways for many years so the promotion of a gene can't explain all of our behaviors. We had behaviors far before we knew about our genes.