The reproductive instinct causes social behavior because the framework requires you to see you as a part of the human species as a whole, with past, present, and future. It's the link between the individual and the collective - you have parents who brought you into the collective whole of humanity and you will create a new individual who will also be part of the whole.
Reproduction is the will to survive, from the perspective of the collective rather than the perspective of the individual. It's the continuation of the species. You intuitively say that the will to reproduce creates social behavior that's sacrificing and protective, but the current trend of population growth could be seen as a cancerous tumor where each individual cell is selfishly multiplying to the detriment of the body as a whole.