I'm sorry but I've never understood this shit. "But, this is who I really am!!" Oh, really? You have a dick. You're not a woman.
Is this a modern phenomenon? If I had to make a quick thesis I would attribute it to the pervasive media and advertising in culture today. Shit fucks with your head.
It is not a modern phenomenon. Though I couldn't tell you all about the history of transgender people myself, I did do a quick google search and this was one of the first links to come up: http://www.transgenderzone.com/features/timeline.htm
- I didn't go through and verify all of the information though, but it might be interesting to do at some point.
I would attribute the thought process of it being a modern phenomenon, though, to the amount of people who are coming out in this day and age - which I would attribute to the tolerance level of society being what it is today. Scanning through that history, there were certain times when people were being murdered for cross-dressing.
Honestly, I don't think that you have to understand it. I can understand how, as a cis-gender person, it would be hard to understand as it's something that you have never had to deal with. The only thing I can hope for is for people to just respect me for who I am, so that I can give the same respect back.