I'm loving it = a kind of slang expression for: I love this so much and I'm really enjoying myself. Of course. "I'm loving it" is grammatically incorrect. But, hey, it works here, doesn't it?

Corporations seem to love this sort of thing: "Toys Я Us" being another hateful example!

The only reason I can say for the difference is to create more emphasis on how much you like ... or in this case love the thing or situation. To show excitement. Hope that helps.

It's just a slang phrase that was adopted as a McDonald's ad tagline. Not sure where the original phrase came from, but it sort of "paints a picture", more active, like Dulcie said, more emphatic, on this verb that is mostly inactive otherwise.