Society seems to classify gender into either male or female, with no other options. Nonetheless, science may allow for something more of a spectrum between the two genders. When researchers study the brain's neural network in action (using instruments like the MRT), they can easily see distinct differences between the male brain, and the female brain. However, when these researchers do the same tests with the brains of transgenders they find the neural networks are somehow between male and female.
I believe that we crossdress simply because it is the way our brain is hardwired. Our brains are hardwired to interpret crossdresssing as actual contact with a female, and it then releases the neurotransmitters that produce the sensations we love. And our neural networks make us all unique. Some of us are content to be a man-in-a-dress, while others need to pass as a woman, and still others find they need to transition. In every case, we want to be authentically ourselves.