Why does it seem everyone likes to pinch my cheeks (face)?

I know that my face is kinda chubby, but still, it's strange how my family members and my schoolmates both do it. Whenever I visit my aunt and uncle, they softly pinch my cheeks with two fingers. And when I talk to my peers at school, they'll suddenly do the same thing and make these, "mmm" noises while they're pinching my cheeks (face). Also, I'm fifteen, and this has happened to me since first grade. There are also other classmates who have chubbier cheeks but no one pinches theirs. What does it mean?
Why does it seem everyone likes to pinch my cheeks (face)?
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