Probably because you're a mostly straight male.
Because she doesn't want you.
Because a lot of lesbians are hot and strong minded (that's sexy).
For some reason, a lot of guys I know are obsessed with lesbians. Something about the girl-on-girl action turns then on, I guess. <br />
You probably want to have sex with her because she's a chick. Being a lesbian doesn't take out an X-chromosome.
You just want the ********* fantasy