I don't know about other writers in the world, but for me, trying to find the best starting place, and the best ending, to a story are the hardest. I find myself constantly rewriting them. Oftentimes, I love the beginning or ending so much I can't bear to start over or work on it more. However, a mentor or editor says,"You might want to redo this part." So, back to the drawing board I go. Yayy...not really.
Coming up with an engaging hook to keep the reader interested past the first couple paragraphs can be extremely challenging. The writer has to come up with something unique, terrifying, confusing or even provocative to cause the reader to engage with the story and the characters. From personal experience, reading the first words of a book is my deciding factor for whether or not I'll keep reading. If I know the author pretty well and already like a few of his/her books, I'll probably continue reading regardless if it's good or not. So, while I realize that this aspect of writing is seriously important, I just wish the task wasn't so aggravating. Sometimes, it feels worse than going to the dentist.
Endings have a different set of hurdles to overcome. I have to satisfy the reader. I could leave them hanging, but I still need to end the story in such a way that despite a possible "Noooooo!" reaction, the reader still enjoys the story--and thinks well of me. Again, this is another part of writing that's absolutely crucial, but is really time consuming and challenging to perfect.
Over all, both of these hardships don't keep me from writing. Instead, they push me to become the best writer I can be. Don't let obstacles or people stand in the way of achieving your dreams!