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.

Part of it is that a lot of adults now never took calculus, because calculus wasn't as commonly offered in high schools as it is now. In their era, it was a college level subject. So they see it as super high level.
People fail in calculus courses because it is at a slightly higher conceptual level than pre-calculus and (high school) algebra. Calculus requires that you put in a lot of work doing practice problems, which is something a lot of people aren't willing to do. 
Ultimately though, calculus is a bogeyman of sorts. It's really not the devil it's made out to be. For someone who did well in pre-calculus, calculus would just be the next progression, and it wouldn't seem like some huge jump up in difficulty. The real abrupt change in mathematics is from computation to proofs. I'm assuming your class isn't proof-based.
So, to not fail, just read your textbook, pay attention in class, and do practice problems. Normal behavior. You'll be fine, really.
