Why is calculus considered to be difficult? I am going to take a calculus course soon and everyone tells me that it is very difficult. What is considered difficult about it and why do so many people fail in calculus courses?I am asking these questions so that I can prepare myself beforehand and do not face any difficulties which most people face.

One of the difficulties with calculus is the amount of time spent solving one problem. Most of that time is simplifying, evaluating, and the like, before applying the calculus-level concept at the end. If you can still do precalculus-level work fluently and without many mistakes, then the exercises won't take too long; it's when students get bogged down by loss of fluency that they get frustrated with how long it takes for one exercise (or exam question).
One other piece of advice that I haven't seen in the answers posted before this one: go to your instructor's office hours, even if you have no specific questions to ask. Just watch as others ask for help (and receive it). Doing that was a huge help for me (though I got a median-level score on the midterm, I got the highest score on the final), so I recommend it for all college math courses, but especially for calculus. 
