To answer the second question first: we’re not. We are not better than animals, because we are animals by every measurable definition. There’s no need for an atheist to try and find an answer to this question that justifies humanity’s special place in the universe, because there’s nothing to suggest we have a special place in the universe. The idea that we do is a lie we tell ourselves, either to make ourselves feel special or because we’re just too enamored with ourselves for it even to occur to us that we might not have special intrinsic significance. Humans may be the most central thing to humans, but that perspective need not extend to anything outside of us.
As for the first question… it’s almost too nonsensical to parse. What animal is the questioner even talking about, to suggest we should behave like? No two animals behave exactly the same, after all. Many behave in manners that plenty of people regard as more ethical than humans, while others behave in ways that we find repulsive. Shall we behave like ants? Like planaria? Or like sharks, or hyenas, or lions, or horseshoe crabs, or flamingoes, or dung beetles, or... well, hopefully you get the point.
But in actual fact, we do behave like animals. We behave like human animals. Part of the behavior of human animals is to experience empathy for other human animals, and to construct systems of morality and ethics that allow us to interact successfully with other human animals in ways that balance that empathy with our own needs. There are clear and obvious benefits to cooperation within human societies, so it’s not even all that surprising that emotional mechanisms which encourage that behavior would arise naturally. Of course, there are also occasionally advantages to acting selfishly and destructively, so it’s also not terribly surprising that such emotions are also a part of human make-up.
See, this question is built around the assumption that “like an animal,” is synonymous with brutishness, selfishness, short-sightedness, and violence. But that’s cultural baggage built on how the phrase is used metaphorically in colloquial speech, and is actually quite divorced from what being an animal actually is. The question is nonsensical because it pretends that the literal definition of the word “animal,” in classifying organisms is identical to the metaphorical usage in describing behavior. They’re not the same thing.
But I know that when people raise this particular question, they aren’t asking “Why do we behave the way we do?” They’re asking “Why should we?” Or more to the point: “Why should we choose the cooperative and empathetic behaviors over the selfish and destructive?”
There are a multitude of reasons to make such a choice. Among them being “because we want to,” and “because it’s usually better for us in the long run,” and “because it’s the nature of humans in general to be pleased by the wellbeing of other humans they care about,” and “because it’s often necessary in order to enjoy the benefits of human societies,” and “because I take pleasure in taking steps to improve my myself.” Those may not sound terribly lofty to some, perhaps even a bit self-serving, but they’re good, practical reasons that anybody who thinks about can see put into practice every day. And are they any more self-serving than being “good” for the sake of avoiding torture at the hands of supernatural entities? Is there any practical difference between suggesting we should pursue goodness because it’s part of our spiritual nature, and saying that we should pursue it because it’s a part of human nature, period?
The fact of the matter is that atheists find all kinds of reasons to want to be better than the worst parts of our nature. Just as other people do. The desire to be better, to do better, may be an urge that religions frequently try to coopt as their own domain. But they’re human desires, as much a part of those who don’t have religion as they’re part of those who do. We may not be better than animals, but there’s no need for us to be. It’s not a meaningful distinction.