I've asserted that it matters how the lives of animals go. Specifically, I think sentience is the specific quality that (some) animals have that give them moral status. Who has sentience, and what is it? The answers to these questions are a bit less clear, and I think inherently so. Nevertheless, I do have some basic thoughts on the matter.
We can think of sentience as the capacity for sensation, particularly sensations that have negative or positive associations with them. It's possible that the capacity for sensation never arises without either something like a capacity for pain and pleasure. But I think it's possible to imagine a being, even a human, who was able to perceive the world, through sensations just like ours, but to whom those sensations were all of equal (that is, zero) value. And it seems to me that there would be no way to wrong or harm that being. It would die quickly, naturally, because it would have no motivation to do anything. So sentience, of the significant sort that I am discussing, is a type of subjective experience for an individual of the outside world, which necessarily assigns (or perhaps perceives) some sort of value to its sensations.
It seems reasonable to assume that sentience emerges based on the complexity of a nervous system. Most animals we routinely exploit (chickens, cows, turkey, horses, dogs, many types of fish, etc.) have obviously very complex nervous systems that rather undoubtedly suggest sentience. It will likely always be unclear where to draw a line for which animals have sentience, and which do not. For example, mollusks and insects may or may not be sentient, and to varying degrees.
There are some tricky points around here. Gary Francione argues that sentience alone is sufficient for the possession of moral rights. These include, the right not to be unjustly killed, harmed, or, as he most emphatically puts it, the right not to be used as property. Jeff McMahan, on the other hand, thinks that moral status increases with the psychological complexity of the individual. So, a typical adult human has greater moral status than a dog, and a typical grown dog has greater status than an inch worm.
For many practical purposes, I'd like to point out that both views give similar prescriptions. Veganism is the proper response to the moral status of animals, whether or not you accept Francione's view or McMahan's. (McMahan has in fact stated that he is a vegetarian, but thinks he ought to be vegan. And in fact, I think he would have little problem consuming honey, silk, or products from animals with sufficiently rudimentary psychological capacities.)
I'll have more to say on this later (spoiler: I mostly agree with Francione). However, I would like to say here that both views get traction from very plausible intuitions. Francione's view corresponds very nicely to our idea of rights as being rather explicit limits. My right not to be murdered does not wane as I get older, and have less life to left to live. And someone else may live an objectively much more fulfilling and worthwhile life than I do, but I still cannot be forced to donate my heart (or even just a kidney!) to save their life.
McMahan's position gains plausibility because of how nicely it fits in with a plausible model of the development of sentience. Just as (to use a common analogy) a single molecule of water is not wet, a single neuron is not sentient. Is an animal with two neurons sentient? Ten? One thousand? Obviously, there is extreme vagueness in the concept of sentience, which is most likely ineliminable. So to have a view of moral status that varies by degrees, and thus contains a corresponding vagueness, seems appealing.