How Do We Behave?

[Napoléon]

First of all, religion is not a good source for morals. The reason is that the morals are not clear - they are extremely muddled. You'd think that a thing like ten rules set in stone is clear. However, the killing done by 'the holy people' continues. Just to illustrate the point; Moses came down the mountain with the first set of tablets. The people had started worshipping something else in the meantime. Moses gets angry, smashes the tablets, orders the unholy statues to be grinded, mixed with water, and the water has to be drunk. After that he orders several men to strap on swords and kill people. So mere hours after getting the first tablets (and I'll assume those had the same rules) he already breaks the 'no kill' rule.
'Yeah, but there are exceptions! People cry. Then why isn't there a clear point by point list of what those exceptions are? Some bible statements even contradict each other. You can really see that when two groups condemn each other, both using quotes from the same bible...

This is just an example. Most rules are like that. This leads to many religious people picking the rules and deciding what is good and what is not. As such, the bible is extremely shaky as a source of morals.

Now, many theists seem to wonder why people would bother behaving nice if there was no eternal glory as reward at the end. Many of them predict a tidal wave of terrible behavior as atheism gains popularity. But when we look at reality, this doesn't seem to be the case. Atheists are not 'worse' then religious people are. The reason is that humans are simply evolved to be generally nice. Why do you think that helping others feels good? It is an built-in reward system from our brains. We are evolved as a group species, and group species have to stick together and be nice. We have a 'see it from someone else's view' part in our brains. It works automatically. When you see someone suffer, you feel bad about it too. When you help someone and it makes that person happy, you feel rewarded too. You see a friend cry, you become sad. This impulse is hard to overcome and is very strong.
Interestingly, there are people who have a specific brain-defect so that this system does not work. Children like this do not return any love at all, and often hurt people on purpose. They are physically incapable of caring for others. These people are sad cases, because they can't mesh in with society at all. They can't understand how people feel, and they aren't capable of being nice to other people - seeing their cases you start to realize how deep this system goes, and how automatic it is - our day is littered with offering little pleasantries to each other.

The reason I'm nice to people is that I could not stand to see them hurt and suffering. And that is a good thing, I''m glad that I, and people in general, work like that.

^ I hate it when someone says perfectly what I was trying to put into words.

This is true, humans aren't solitary animals, we are social creatures, and therefore we have evolved the ability to be empathetic. This is good basis for human's coming up with moral codes on their own.