I think it is vital to enjoy the creative process to get the most from it. That doesn't mean it won't be difficult at times. You will feel like swearing (and may do so!) when the words don't seem to flow so easily as they usually do, but if, generally, you love what you do as you create your characters and stories, then you can know you're on the right lines. That love will keep you going (along with support from other writers whether that's online or in a good wriitng group) throughout those tougher times and the inevitable rejections we all collect as if they were going out of fashion. The one great thing is I can't think of any writer who hasn't felt in need of that support at times. You are definitely not the only one!
It is difficult to say what my favourite part of the creative process is but I do feel elated when I know I've got a character right. I know then that what I get them to say or do is right for them, and their story will flow much more easily as I am writing from deep knowledge of who and what and why they are the characters they are. I also think that will come through in the writing itself and make for a better read for future readers. I also feel elated when I've got a first draft down as I know I've got something to work with then (and it can only get better! All first drafts need work, even Shakespeare's would have done!),.