Why are best friends so critical towards each other?

It’s like nothing is good enough for them.

One of my friends (Let’s call her Mariah) did a little break dancing... well sort of. Mariah did a spin on the flow. I was AMAZING! I clapped. Her best friends (let’s call her Amanda) was like, “You call THAT dancing (laughing)? I’ve been better moves then that.”
Then Amanda turned to me and said, “Are you BLIND? It wasn’t that good.”
Then Mariah said, “If it isn’t that good to you, let me see YOU try (laugh).”
So she did and I have to admit she is a better dancer. Friends should be encouraging not laugh at each other and criticize each other. I’m surprised Mariah wasn’t upset with Amanda laughing at her. I would have gotten mad.
I’m worse than her and people laugh at me for my awful dancing. Even my best friend bluntly said I am a bad dancer and I would embarrass myself on a dance competition. She said, “Don’t do it. You’ll make a fool out of yourself. Just saving you from humiliation.” She didn’t have to diss me. I’m wanna be on a dance show. 😭😭😭😭😢😭😡😡