Answering questions regarding why we ought to care about anything can be quite difficult. From the perspective of our universe as a whole the intentions of a few self aware lumps of water and carbon, inhabiting a single pale blue dot, orbiting one of hundreds of billions of stars, in just one of billions more galaxies, couldn't be more trivial. However, to us, the inhabitants of that pale blue dot, those intentions determine our very existence.
A great deal can be gleaned about why we act the way we do from evolution by natural selection. In a large portion of social encounters in which we can expect a large number of future interactions with another individual, the strategy that will yield the best expected outcome for ourselves is a tit-for-tat strategy, which means we act generously in our first encounter with the individual and then in all future encounters act toward them the same way they acted toward us in our previous encounter. Members of past populations who happened to possess genes predisposing them to using this strategy were more likely to pass on their genes to future generations.
While evolution can do an excellent job of explaining why we act in the ways we do, it has nothing to say about the way we ought to act. On some occasions our own interests will come at odds with what is most likely to propagate our genes. Maxing another character in WoW is unlikely to improve your chances of reproduction, yet people continue to attempt this task by the millions.
Many people choose to act in whatever way they feel is most in their own self-interest. Often times self-interest will still include some cooperation, but the ultimate goal is still the most personal benefit. As an individual making a personal decision for oneself, this decision is quite understandable, but as a conclusion on how we ought to act, it seems to place an unjustifiably high importance on solipsism. For each of us, our own interests are what is important. We also have every reason to believe that for everyone around us, their own interests are held in that same high regard. If we ignore the solipsism for a moment, the decision that would be of the most combined importance between all lumps of water and carbon would be to act in whatever way we could to maximize the fulfillment of interests.
Humans are by no means the only species with interests. If you have ever seen a baby pig squealing and writhing as its tail is being docked, you would say without doubt there is an animal expressing a strong interest. Similarly, the pig likely has a similar interest, that it is not yet aware of, in not suffering the medical consequences of leaving the tail undocked in factory farming conditions.
If we are to live up to our goal of acting to maximize the fulfillment of interests, then this cannot include only the human species. In the case of non-human animals our efforts can often go much farther than they would with humans. For less than a cent donated, Vegan Outreach estimates they are able to save a life. Compare that with the roughly $100 it would cost to save a human life in some of the poorest countries. This is still a tiny amount when compared to the tens or even hundreds of thousands of dollars many people expect in medical care in first world countries.
Many humans still suffer a great deal of pain which could be treated, but in comparison to the lifetimes of confinement and often torturous cruelty most non-human animals endure in today's factory farming conditions, this human suffering is a walk in the park. When combined with the relative simplicity of merely changing our diet, veganism clearly becomes an ethical imperative.
If you have not yet read the Why Skeptic question to our blog, I recommend doing that now as well.