If you are asking for reasons 'why should a person be good?' then the answers might depend on whether you ask for the reason/sense of being good, or the cause/advantages of being good.
If you want to explain why being good is... good, then the answer might be different than the answer to a question 'why should a person be good' because this second question usually leads to persuading the said person that being good is better, that it has many advantages, that not being good might have consequences etc.
Basically one question is about reasons, the other about persuasion.
If I were asked the first question, I'd say that good/love/God make sense and bad/hate/atheism logically have no sense, so good is a reason by itself, while bad is not. It's hard to explain, but basically having sense/reason requires sanity. There is no sense/reason without sanity. And I believe that good has sanity by itself, because good is God himself. So good has a reason. Bad has no sanity, so it cannot have a reason.
This explanation without referring to God would be like referring to football without referring to ball, so I guess if I were to meet your request for no such reasoning I'd have to say nothing at all. But that's basically because I believe there is no such thing like reason/sense without God, so asking about reason would be unreasonable. Of course if you do not agree, simply ignore this paragraph.
If I were asked the second question, my answer is three posts above. You asked for no reference to God, ok. This question can be answered without reference, because it's not asking about reasons, so the answer does not have to make sense. It only has to persuade a person to be good.
I hope anyone understands what I meant, because I only vaguely understand myself