Ask HN: Why is calculus considered such a difficult subject to learn?

I see this primarily from students in US universities. They dread their freshman calculus classes? Why? Are the teachers bad or the curriculum or books?

Calculus is such a beautiful field with geometrical explanations for almost every theorem. It is the least abstract or dry subject in math.

What is your favourite textbook that made you understand and love calculus (single and multivariable)?