Why be good?

[Randy Carson]

If there is no God, then there is no hell; and if there is no hell, then there are no ultimate, eternal repercussions, good or bad, for how we live out our mortal lives. Of course, atheists insist that people should be "good without God."

But why? If God does not exist, why be good?

[Minimalist]

So you don't get your sorry ass thrown in jail by secular powers who are actually real.

[Dystopia]

Define good and evil and then we can talk about it

[Longhorn]

Another 'if there's no god where do your morals come from?'?

Look, it's really simple: just because we don't subscribe to your particular mythology does not imply we lack empathy for fellow humans. That's stupid.

[Salacious B. Crumb]

Define good and evil and then we can talk about it

That’s a good start, because a bible believer’s standards of good and evil are generally off the mark, compared to people who don’t base their morals off a book written by violent, judgmental assholes.

[ignoramus]

You get your morals from an obsolete book, we get ours from the book of common sense.
Common sense isn't as common as you may think.

[Esquilax]

But why? If God does not exist, why be good?

Please circle which of these two human cultural sets you would wish to live in:

A: Modern society.

B: Mad Max world.

Being good nets you option A. Not being good nets you option B. That's just logically true: human society has been built, from the ground up, by our cooperation and the establishment of our social contract. Literally every single thing that you enjoy in your life stems from those two things, including actually your life too, in all probability, given that the medical science that extends human life is also a result of cooperation. It's so very strange to me, how many theists ask this question, as though they assume human society requires no upkeep or buy-in from actual humans.

Also, have you considered the consequences of what you just said about yourself, there? If you're seriously asserting that there's no reason to be good beyond the threat of hell and the promise of heaven, then you need to realize that you're only apt to talk about yourself, not everyone else: all you're really saying is that you can see no reason to be good beyond a carrot and stick. Essentially, that you would not be a good person if you didn't think you were being alternately cajoled and threatened to be one.

Do you really think that about yourself? Do you hold yourself in that low esteem?

[pocaracas]

Because we live in a society where cooperation is key to the survival of the whole society.
Cooperation can only be achieved by not being bad, aka, being good.

A non-cooperative individual is set aside and his survival is imperiled.
Sure, nowadays people are not supposed to die in jail... But passing through there tends to keep those individuals in a behavior that systematically lands them in perilous situations... Eventually, they perish while society moves on.

If the grand majority were to not cooperate with society, then society itself would crumble and the survival of the species would become problematic... Luckily (or not), that's not in the cards for the near-mid future.

[The Grand Nudger]

If there is no God, then there is no hell; and if there is no hell, then there are no ultimate, eternal repercussions, good or bad, for how we live out our mortal lives. Of course, atheists insist that people should be "good without God."

But why? If God does not exist, why be good?

So, I gather that you require magical chants and miracles in order to muster up the will to be "good", lol?  It's easier for me, apparently....... it's just who I am.  But if you're the kind of dick that would be out skullfucking babies and pushing old women into traffic were it not for your silly stories, I'm going to insist that you continue to attend mass regularly.  Might want to get some help, too.  

[Iroscato]

Because the alternative would in all likelihood lead to the extinction of our species. From a coldly biological perspective that is the foundational reason. We know not what lies beyond the edge of death, and whether or not there is some supernatural being keeping tabs and taking notes on us all - no human being alive can know this either way for certain.
All we know for certain is that we're here, and our situation is what we make of it. We are stronger when we cooperate, we are stronger still when we unite, and we are at our strongest when we love. Genetic traits intermix with social constructs and form our civilisation as it is - though I'm of course talking about western civilisation, having been born and raised in it.
Many cultures across the world and throughout history have lived without the need to worship a god or gods, and many have been every bit as 'moral' as we are today. Others have worshipped gods and engaged in warfare, ritual sacrifice and slavery. It is a result of the multifaceted nature of humanity by which these different paths are taken.
To stress another point, and one that has always been at the heart of this question - are you saying you are only moral because you think you will face judgement for evil acts, and that's the only thing stopping you? Really ask yourself that, and be honest with yourself. You may find you answer your own question, for better or worse.