What's different if I change "the" to "a"? (a real thing vs the real thing)
Remember that "the" is used more often to refer to real nouns or specific things. In this case, "the real thing" is an actual expression, meaning something authentic or real love. This is something specific, so "the" has been used here. "A real thing" would not work in this context because it is not the actual expression, and it also refers to something more general. Ex: This is the house I grew up in. Ex: I'm going to eat an apple.