Why do people believe animals don't have souls?

I am not religious or anything, but I think that they do have souls. When my lab died in November (I had known her all 12 years when she died-- 13 now), I always knew that she wasn't there. Even my granddad, a priest, belives they have souls, and I think that all the people who say otherwise are wrong. The love given by dogs and cats can only come from having a soul. Otherwise, they'd be drones.