Why 'should' atheists be moral?

[Mystic]

The best motive should be belief in the value of fellow human beings. My personal belief is increased with the pantheist notion that everything is made up of God who is the ultimate value or the comprehensive station of value, the all-value, but I don't see why Atheists can't believe in value of humans. If you believe you should value yourself, you should value human beings as well. Although I don't think value of living beings can exist without supernatural as a basis, I don't see why Atheists who believe value of themselves can exist without God don't need to believe in the value of others.