Why Do People Like to View Themselves as Middle Class So Much?

Genuinely asking... it seems that the default assumption of nearly everyone I've ever had a discussion about economic background with is that they come from a middle class family/community. These peoples' economic backgrounds vary WILDLY, however. I'm currently talking about this with someone who comes from a family where almost everyone has a masters degree. They lived in a nearby suburb that's known for its wealth and went (still go?) on family vacations abroad with some regularity. I recently had another close friend who grew up on and off food stamps say that she came from a middle-class household. These two definitely grew up in very different environments, but they defined themselves in the same way when speaking with me.

So... what's the allure of the middle class? What even IS the middle class? Thoughts?

ETA: Relevant and awesome interactive infographic courtesy of Chasmosaur100.