The people you're talking to don't really know anything. Lots of studies have been done trying to find the common factor in the way transgender people were raised-- there isn't one. Plenty of people try not to be transgender, they choose not to be transgender and, guess what? They're still transgender. Whether or not to pursue hormone therapy, surgery, and transition is a choice, but sometimes the alternatives are pretty unthinkable. Yes, sometimes (very rarely) gender identity changes during a person's life-- sometimes weight changes, or eyesight, or hair color, or mood. The fact that it changes doesn't mean its a choice.
Whatever causes us to be this way, it's something intrinsic to the individual, which is either set before birth or by some random environmental event in the first couple years of life. People can believe anything they want, they can say anything they want, but that doesn't mean their beliefs are factually correct.