Some words just don't flow off the tongue. Like they sound great and pretty when read, but when you're pairing them with other words within a certain rhythm...just does not work at all. And so frustrating trying to find a synonym for a whole phrase that you've already become attached to.
Also, it's about finding a rhythm for your words that not only work but convey the right emotion. You know, putting the right emphasis on the important words and delivering lines in a such a way that creates a certain feel.