LOL, if doing it didn't make us happy, I don't recon we'd be here in the first place :-)
I think for me, it comes from the simple joy of seeing someone in the mirror that I instinctively recognize as "me". For most of my life, I didn't even know I was missing that.
I'd fumbled around since I was 7 or 8 years old, trying on my mother's clothes ... later on nipping things out of the pile on the way to the thrift store, etc etc ...the usual sneaky stuff nearly all of us did as children. I didn't even know why, I was reaching toward something and I didn't even know what it was. Until one day, in my early 30's, the final piece of the puzzle fell into place. For me, it was hair. I put on a cheap Halloween store wig, and it finally clicked. No makeup or anything else ... I looked ridiculous for sure, but there was that instinctive recognition ... "that's me, the girl that's always been inside me, trying to get out ... that's ME!"
That was the first time I ever felt that simple joy, of seeing a reflection in the mirror that represented who I felt like . Of course I was all smiles! Still am, when I get to see her! :-)