Because as we (most people) see ourselves as vulnerable, we can see/sense it in others. It is called, compassion. Compassion is not necessarily a byproduct of having faith in a god, although, it can be for some people. It stems from our own vulnerabilities perhaps, and we see others with those same needs, wants and vulnerabilities. This doesn't mean all atheists will respond to others' needs with compassion, or concern. It simply answers your question as to how an atheist is capable of finding compassion for others, despite the fact that he/she doesn't believe in a god. Stripping away the labels -- we have more in common than not.