Why to behave morally when there's no objective good and evil?

Live morally for the harmony of the society to which you belong. To do so is evolutionarily beneficial.

You are forgetting one thing: Consequences.

Just because there is no intrinsic morality does not mean that there are no unfavourable outcomes. For example, in a world without intrinsic morality, putting your hand on a stove will still cause you pain. Even if it wasn't "immoral", does that mean we should keep touching the stover over and over again? I don't think so. There are many things that we can do that end up feeding back to us. Moral codes were created to ensure a stable system in which unfavourable outcomes were avoided for as many people as possible. In other words, if you don't want to get burnt, behave.

So basically, why would that person play by the rules if he decided there was no rules.

We are little humans, on a little tiny planet, living in our little tiny homes, in the middle of GIGANTIC universe. I don't think the universe gives a damn about good or evil, so of course it all comes down to what we perceive.

Just because I recognize the fact there is (I hate using highly definitive terms, because to me, there is always a little room for doubt; who am I to say "what IS" so I use that term lightly here) no ultimate good or evil doesn't mean that I can't or don't want to be an overall good and constructive person.

Basically what I am trying to get across is this: just because no ultimate universal good or evil exists doesn't mean I have to abide by it. Sure, it doesn't reeeeaaally exist, but I am going to make it exist, because I am a person and I perceive good and evil in the world. Suck it, cosmos.

Even though you may think that good and evil are not objective values, you can still have emotional reactions to things so that you will find some things unacceptable, e.g., a situation where someone is treated badly by others.

It may also be that while you believe, intellectually, that there is no objective good and evil, you have not entirely taken it to heart to an extent, so that you still have principles/ideals that you don't question to the point of nullifying them.

Compare it to taste, as in taste in music. Many will agree that taste is subjective, and that things you like may be disliked by others. In other words, there is no objective taste. You could then ask, why would someone who does holds disbelief in the objectivity of taste in music, e.g., prefer one piece of music over another. Or, does a particular piece of music have any value at all, since there is no objective value to it.

I think humans have evolved to have capacity for empathy and compassion, and I like that. I think compassion and empathy are good things. That can be reason enough.

If you believe there is no good and evil, you believe there is no moral and immoral. Therefore, there's no such thing as behaving morally or immorally - at least in an objective sense :P There is no "moral" or "immoral behaviour", just "behaviour".

Because you feel more fulfilled by behaving in a way you believe is good. Being bad doesn't mean having more fun or being more fulfilled or more happy. Amoral doesn't mean antisocial.

Because if if behave in a manner deemed immoral you may just find your ass in jail.

That depends on what you mean by "moral". Surely you realize that's just another word for "good".

Because a shared moral code is fundamental for our society to function like a well-oiled machine, a machine which serves to protect and preserve our species.

Simple as that.

If we all threw morality out the window, we'd die out pretty rapidly.

Think of morality as an internal rule-book that we've evolved to keep us alive and well.

It's a natural and inherent aspect of humanity - part of our biology.

Not everything you read is true.
There are many things that are neither good nor evil, but there are things that are.
No matter how you look at it doing something like putting a baby in the microwave is evil.

Because it's the right thing to do? And I am not willing to waste my time with cop-out arguments asserting absolute moral subjectivity.

The golden rule really does make a great deal of sense if you stop to consider it. Hell, it very well may be the only "commandment" you ever need.

Regardless of how you perceive morality, there are consequences for doing things that others perceive as bad, hence laws. You behave in a moral fashion because otherwise you'll go to prison.