There's the element of self-satisfaction. Acts meant to reinforce one's own feelings of being a good person.

Empathy, self superiority, boredom, personal fulfillment, low self esteem. It depends on who they are helping with what and the power dynamic between them.

Usually people do expect something even if it isn't something tactile and goes unsaid. Plus it just makes you feel like a decent person.

So, obviously you are not doing that, or you would know the answer.

survival of the species, to begin with. altruism developed along with our brand of sentience, to continue, and that is of course an extension of the survival of the species but has reached beyond it, perhaps.

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Well, I think we gain a lot more by helping each other.

They do expect a reward - it's called feeling good about yourself. Just b/c it's not fame, or recognition, money or positive perception by others (although I would argue at times it is these things), don't assume there isn't a selfish reason for being nice and helping.

Because it is part of being human.

I would refer those interested in a detailed analysis of this question to the book, "The Selfish Gene" by Richard Dawkins.

Because we're a social species and as such altruism is built into us and has many benefits, both tangible and intangible.

Altruism, living for others, and sacrificing for others as a moral good is anti-individual rights. Comte, the philosopher who coined "altrusim", says, "[The] social point of view cannot tolerate the notion of rights, for such notion rests on individualism. We are born under a load of obligations of every kind, to our predecessors, to our successors, to our contemporaries. After our birth these obligations increase or accumulate, for it is some time before we can return any service.... This ["to live for others"], the definitive formula of human morality, gives a direct sanction exclusively to our instincts of benevolence, the common source of happiness and duty. [Man must serve] Humanity, whose we are entirely."

Why is that the good?

But there are always rewards when one helps others. The entire world benefits from a single act of kindness. Granted, not much from a single act, but just imagine what might happen if each of us performed just one every day.

Seems their way of contributing something to humanity.. Many contribute in ways not always apparent. I’ve found the best of humans to be generous and nurturing, and the worst - the opposite ~

The reward is having done something kind

Because it's the human thing to do.

You always get rewarded when you help someone, you just don't ask for it but do it from the goodness of your heart. The joy of helping is in the reward, because you know that you had a part in that.