Because suffering is a facet, perhaps even the salient quality, of sapience. Awareness of one’s ephemeral existence, awareness of absurdity, particularly that of pain’s pointless pangs, are attendant to any sufficient self-awareness. Ignorance is bliss, and who are we to deprive another class of being of its succor?

That said I doubt we’ll be able to breed canine buddhists for at least another millennium. We may (probably) leave aside such qualms until then.