As a matter of fact, there are many reasons for this question. It varies from person to person.
Sexual gratification
Sexual gratification is the most common reason. Crossdressers may feel a sense of sexual excitement when dressing a female clothe and feel the soft lining material. They may especially love the bras. Seeing the female image of himself in the mirror could trigger sexual impulsion.
Feminine expression
Another reason for a man crossdress is that they want to express their side of female. Many men feel that there's a woman living inside of them. Occasionally "she" wants to get out. Boys are raised to be strong and brave. It seems that if there's a man who often cries, or show his weakness, he would be considered as gay. But there's no such perception when it comes to women. Therefore, some man wants to show their weakness and want to be free to express their feelings, they would crossdress to be a female to achieve that. For some men, the heavy responsibility and pressure can be casted off provisionally by crossdressing.
Female power
One of the reason for men crossdressing is to feel and experience the power of women. What is this power? A beautiful woman catches the attention of all men in the room when she walks in. This is power. The development of lingerie model industry also demonstrates of the power of women. Not only men, but also women prostrate themselves to this power. It gives rise to hundreds and thousands of visual enjoyment. All men are obsessed with this power, which is also one of the reason for human reproduction. Hence, crossdressing seems to be the most intuitionistic and direct way to feel this power, or at least for a very short time, to understand what attribute to this power. This situation may be similar to those who wears expensive shoes and clothes, sit in front of a huge desk in over 20th floor and overlooks the whole city and pretend to be a CEO of a company worth over millions. There are people open doors for him, buy him coffee, send him home after a gathering, etc. These will happen without him saying a word. The power is pretty strong and ravishing.
Sexual capability of women
Women are pretty good at sex. All men and women know this fact. You only have ot turn on TV, read a magazine or watch some show, you can realize it clearly. Sexy, delicate and beautiful are their labels. Men craves for these qualities. Some may choose to mate them, while others may want to experience it in person. Even it is just for a day can give them some excitement.
Unfortunately, crossdressing is still considered as taboo in many countries of the world, especially for male. Men who crossdress in public often gives rise to mockery, malicious comments, even violent behavior. Maybe years later, there will be more rights and equity for crossdressers. Just wait and see!