You can get it if, say, you've been burned and it lead to a disfigurment, or you were in an accident of some other kind.
Apart from that, the only way you'd get it free is if it was having a "significant psychological impact" on your life. Like, serious psychological impact. Oh, and trust me, they have people who understand issues like this, and would be able to tell whether you were just putting it on to get it for free.
If it's weight loss surgery, they'd make you see a dietician, and any other surgery they would arrange counselling sessions first to try and solve your psychological problems. What I'm saying is that it's more than going to your doctor saying "my boobs make me sad, can I have bigger ones, please?"