How can I get my insurance to pay for my braces?!

I need braces on my lower teeth. They're not real bad but are getting a little crowded. I have sensitivity but I can't think of how this could prove that I need braces to my Dentist or Insurance. What are the most important reasons an insurance provider would need in order to pay for braces and how do I get started with telling my dentist? Would the sensitivity be limited to where the teeth are crowding or no? Could jaw pain be a reason? Thank you.