Self-sacrifice happens among other species as well, though we don't notice that or consider it normal. Several animals for example sacrifice their own lives in order to reproduce and ensure the survival of their offsprings. They protect their young at the risk of their own lives without hesitating once. Compared to animals humans are still mostly egoistic in nature. Though that is because animals still act on instinct and humans consider their options. Those willing to fully give themselves up for another are rare, but admirable. As far as I came to understand them, strong emotions are at play in these cases.

[It happens among nations as well, but he refrains from mentioning that. All his siblings have given up on their existence to make him who he was today after all.]

As for pets... No. They don't fight. At least most don't. They are companions. Maybe you should try keeping a dog and you may find out their value.