What do the doctors look for? Is there a chemical imbalance in the body? Like a man is producing far more estrogen then what is normal or whatever? And if so wouldn't it be easier and less dangerous for them to take medication so that they are comfortable with the gender they really have?
it's differences in how the brain is wired
we know there's a difference, we can see it on brain scans, it just hasn't been pinned down what exactly is causing it or how these differences work together to create that sense of "I don't belong." It's a very complex condition that for the most part, is unique to individuals in exactly how it forms and manifests itself, although there is that general sense of for example "I'm not a female. I don't belong."
There is no medication that is known to be anywhere near 100% effective for it. All people who want to actually make a transition are first put through a psychologist so they can tell if it can be solved through other means, like anti-depressants or psychological therapy. If that doesn't work then the best treatment option is a transition, which has a very high success rate (Success being that they're comfortable and happy with their body)