Ah, the "reason" argument. Are you gifted with the ability of telepathy, Norman? Are you so absolutely knowledgeable about the inner workings of a pig's mind? You can't even tell for sure that other people are sentient, aside from the fact that you know yourself to be sentient, and make an assumption that things like you must also be sentient.
Basically, it works out like this:
If I, my sister, my best friend, my dog and a stranger are in a burning building, and all of them (aside myself) are incapacitated, I will save my sister, due to familial connection.
If she can get out on her own, I'd save my best friend, because, well, he's my best friend.
If both my sister and my friend can leave under their own power, I'm saving my dog, because I know him and I have an emotional connection to him.
If all three of the previous can escape under their own power, then I'll help the stranger.