People who believe in a secular basis for morality usually look to our social needs and interactions to give us a reason to be moral. These motivations are legitimate, but may not hold up too well under the following circumstances.
1) People become convinced that appearing moral is all that is needed, as opposed to being moral.
2) People are in a position of control over others, and therefore do not consider their behavior toward those they control governed by ethics.
3) People just do not feel any need for social approval. Socially isolated persons often end up being serial killers, for example.
In these three situations, I do think the ordinary "social" reasons for leading an ethical life wear thin.