The reason

Four months had passed since the events of the Great Shift. Matt considered himself lucky to be shifted with a guy of his same gender and age, much of his life kinda stayed the same.
For Jeremy on the other hand life was vastly different as he was shifted into Monica, one of the cheerleaders at the time, believe it or not. 
Matt had seen Jeremy eating more and more food, seeing his body getting bigger and bigger. He had been still about it for so far but he felt he had to know.
"Jeremy....err...Jennifer", Matt corrected himself when she eyed him.
"Why are you so......uh..big?", he lingered a little bit on the last word. Jeremy/Jennifer was a lifelong friend of Matt so he was pretty sure the question wouldn´t be ill-recieved.

She began to explain: "You see, I will be a woman for life and the former occupant of this body was an excersise freak and thin as a stick. So I figured that why shouldn´t I embrace my femininity all the way. Like in getting some meat on my bones. Being as flexible as when I was shifted was nice and all but it doesn´t beat the feeling of having someone looking at me with desire. It makes me come to terms with being a woman."

"Figured it was something like that. You seem comfortable being a woman", Matt said.

"Yeah. I´m slowly getting there",  and with a sly look on her face she added: "There´s only one thing left to do. Having sex with a man".