AllieSF is right on the money. It's a spectrum. People who like the softer silkier fabrics of women's clothing (until recently men's underwear was like sandpaper, chafing and rubbing in all the wrong places. Wearing panties just for a break from that awful feeling is a reason to crossdress in itself), people for whom it is a sexual experience, people who don't like to be told what they can and can't wear and like to decide for themselves, people who have a stronger than usual feminine side that needs to be expressed and clothing helps with that, people who just feel more comfortable in women's clothes, people who have a duality of male and female, and people who are women mistakenly put into male bodies. There are hundreds more reasons that I haven't listed.
Myself, I'm on a journey to discover who and what I am. For many years I repressed the feminine side of me only allowing myself to crossdress for fetish reasons and feeling really guilty about what I had to hide but couldn't live without. There are medical factors to be taken into account in my case too. As it stands I feel that I am half male and half female. Each cannot survive without the other unless I want to go back to living in misery and denial. Do I want to become a woman? That is what I need to find out. It's different for everyone.
Your boyfriend may well just like dressing for the sexual thrill. If it goes deeper than that, then you've probably got a guy who will love you and treat you right since he will be sensitive to how you feel, and it doesn't mean the end of your sex life at all.
