I wonder why cats and dogs don't get along most of the time.

I know you can train them to like each other or at least get each one to get used to being around the other. But why don’t they just naturally get along? I know it sounds like a silly question but it really got me thinking. Maybe it’s because they’re such separate species. Dogs seem to have this aggressive instinct to be the pack leader and defend themselves and cats seem to have an almost opposite instinct of survival at all costs. It would be interesting to get inside a cat’s or dog’s mind and see what they are thinking of.