I think "why be good" is perhaps the wrong question.
Instead, "why
are we good" is the right question. Because clearly, we are good, by enlarge. Atheists do not in general go around murdering and maiming. In my life I've known almost exclusively atheists and they have almost all been good people. Some Christians talk like atheists "would murder people all the time". Well, they don't. So clearly, that statement is wrong. It's more a projection of what they think
they themselves would do without the fear of God, and that is the telling and scary part. Fortunately, their fears are usually not founded regarding themselves. I've heard many people say exactly this, then be astonished after they lose their faith that have no desire to do immoral, horrible things. They have been programmed to think that way, regardless of all the evidence to the opposite (atheists are not maniacs). I didn't say all atheists are not maniacs. Life is not binary. There are plenty of shitty atheists. But if you look at atheists and theists side by side, there is nothing to suggest theists are actually more moral by any meaningful, real standard. It's about the same. In other words, it's irrelevant. It may be the case occasionally that religion is enough of a psychological program to hold a genuine dangerous psychopath in check, that is some small consolation for all the untrue propaganda.
So why are we good? The simplest answer, I think, is evolution. Those who have been good at cooperating and at caring for society as well as themselves have fared better, so it has been promoted by natural selection.
Speaking personally, I appear to be hard wired to not only not want to hurt people, but to be actually physically unable to in normal circumstances. I can't even step on a fly when I think it is the right thing to do. And I have a very strong sense of inner morality which makes me my own harshest critic.
Theists often like to think morality comes from God. It's nonsense, or at least it's fully undemonstrated. Look at the evidence. This forum is filled with people who have de converted. Did they turn into maniacs? No. Is there evidence of large numbers of deconverters going mental? No. So again, human nature is to generally be good. To deny that is to deny reality.
It is obvious theists get their morality from the same source atheists do: themselves, ultimately. Of course we learn some of our morality from our parents, from others, through our experience, and by having empathy. This all shapes us, along with our evolutionary tendencies, to generally give a fig about other people. Not everyone does, things are not that simple, but most of the time. If you stop a random person on the street and ask directions, probably 90% of the time you'll get a helpful response. There is virtually nothing in it for them, besides the satisfaction of helping someone else. So there you go, another reason: it feels good to help others. And generally, it does not feel good to harm others.
Theists do not follow their own morality that they preach. They often claim to have "objective morality" and have "the word of God" yet if you examine these things, they are both flawed. I've read the bible, I know what it tells you to do and how to live your life. Most Christians ignore most of it. They have to, or else they'd be dead or in jail. Sadly most theists are quite used to the kind of mental gymnastics necessary to convince themselves that they are following it, even when they are doing the exact opposite.
So what is really happening? The theist is doing what they think is moral from their religion. If they think something is immoral, generally they'll find a way to convince themselves that the religion "doesn't really mean that" or "that doesn't apply anymore" or any other number of excuses which seem absolutely ridiculous to atheists. I'm thoroughly glad that theists do override their religion this way, or else you would all be absolute psychopaths. Seriously. Look at ISIS, that's an example of people really practicing what they preach. And the results are exactly what I would expect: Bronze Age style barbarianism.
So basically theists use their own morality. They feel righteous when it lines up with their holy book, and they make excuses when it doesn't. So they are using their own morality to filter their holy book. In other words, they are just using their own morality. Now sadly, sometimes a broken and horrible bit of immorality from the religion will make a good person do something bad that they otherwise wouldn't if it weren't for the religion. This is the big problem. Religion can make morality worse by poisoning ideas with outdated, ignorant hateful messages. Like I said most people ignore them, thankfully.
So, Randy, do you really think the only thing keeping you in line is an entirely undemonstrable celestial being's threats? Does it give you a clue that religion
needs to make such horrific threats in order to make people subscribe? Can you think of anything else that is obviously true that requires threats to get people to believe it? Even for those who don't believe in "hell" still generally believe in "heaven". So God is holding your soul hostage. Do what I say, or the soul gets it. This is more of a threat than a reward.
Atheists are capable of being perfectly moral, and most of them are. So studying why that is will hopefully answer your questions
The cop out is always "well God makes atheists moral". Well, whatever. Guess what, it's yet another unfalsifiable and unhelpful totally pointless claim. If you are predisposed to just believe God is responsible for everything, then this is how you will rationalise it. I actually think God (in the bible or the Quran) is the most vicious evil cunt I could imagine. If he was real, I would hate him. I am certainly not "in his image", I'm about exactly opposite to him. Yes, I'm more moral than him, way more. So is almost everyone.