Why do you think Stephenie Meyer has Bella fear dancing, blood, and needles?

I think it was for irony.

She hated dancing, yet fell for a super graceful guy who could dance with ease, and she would later become as graceful.
She hated blood, yet she fell for a guy whos life revolves around ingesting blood, and she would later revolve around blood.
I don't remember her fearing needles...

They are minor obstacles she overcame, which demonstrated ways that being a vampire was the best choice for her.

Irony was already mentioned, but I think that the fear of blood and needles was added so that Bella would appear madami fragile and vulnerable. Fear of dancing is just a part of her clumsiness. Many people REALlY dislike dancing, simply because they suck at it (I'm an example).

its because she has a symptom, where she seems to be clumsy and trips over her feet every two seconds, if possible