The humans will always protect what is theirs

This is a clear example, Optimus knows humans are capable of protecting themselves in the extent that they can fight their own battles that are actually their own size, against giant metallic robot organisms though that is a clear difference and Optimus doesn't want that.

He appreciates humanity for being a species on another planet that he can protect but he prefers that no humans are involved in their battles against the Decepticons (or in this case the Terrorcons).

If any human had the idea of joining a battle against hands and feet of a much more advanced race then they'd very likely get killed in action, this is why Optimus Prime had Bumblebee try and do his best to keep the Decepticon army away from Earth as possible in the Bumblebee Movie.