Can we move from an "is" to an "ought" - from some set of objective truths to some set of subjective reasons to act in a given way? In my view, theists and atheists are equally over the barrel on such questions.
Theists (especially traditionalists) often claim that divine commands are objective in the sense that they do not vary from person to person or by time and place, but this sort of objectivity does not help one get to an ought without adding in certain assumptions such as one ought to please the gods for reasons X, Y, Z. Inevitably, such reasons must be rooted in the subjective desires of particular moral subjects in order to provide people with a good reason to act as they ought. This might help explain why so many mythologies have created incredibly paradisaical and/or shockingly horrific settings for life after death. Ordinary earthly carrots and sticks just weren't enough to make divine commands relevant in the here and now.
Atheists (particularly moralistic humanists) also have to make unjustifiable assumptions about empathy or compassion and have faith that other people are motivated by the same concerns that they are. Since private charities aren't exactly raking it in - compared to, say, Hollywood studios - this is not exactly a winning bet. Probably people are more concerned with their own happiness than with an idealized and universal sense of empathy for fellow humans.
Ultimately, there is no way to answer the question "Why ought I act or refrain from acting in this way?" without referring back to one's own subjective desires. I sincerely hope that your own desires are guided by empathy and compassion, but if they aren't, well, about all I can do is have you shot.