Hello, Cannonkuo, and welcome to the Grammar Exchange.
The slogan of McDonald's has been puzzling me since I heard it on TV long ago.
Why not just say "I love it" instead of "I'm lovin' it"? Any special meaning it it?
Normally, stative verbs, such as "love," are not used in the progressive. However, "love" and related verbs of liking and disliking are sometimes used in the progressive when attention is confined to the affection in the immediate present. Compare: "I'm liking the class so far."