* I like slash because it's something I can't get from the source text. Much of popular media depicts m/m relationships that have an intensity that could be interpreted as sexual but for the presumption of heterosexuality. The presumption is such a strong one that it guides much of our interpretation, as famously expounded upon in Leslie Fiedler's essay "Come Back to the Raft Ag'in, Huck Honey!" Slash takes away that presumption and allows for interesting possibilities it prevented.
* I like slash because before I ever heard of it I sometimes saw what seemed to be sexual desire or connection between male characters in the media. I saw it in Butch Cassidy and other buddy movies (and vividly remember Paul Newman being interviewed and surprising the interviewer by acknowledging that yes, there was a sexual chemistry between him and Robert Redford in the movie). I saw it in Star Wars, Episode I and I had not heard of slash when that film came out. I also didn't know (since I didn't read secondary canon, where it is explained) that when Obi-Wan and Qui-Gon look at each other intently and then spring into action they are supposed to be communicating over a limited telepathic link that binds master to apprentice, called the Training Bond. Well, where I come from when two guys look at each other like that they're not planning a battle. I was happy to find Qui/Obi slash and the Master-Apprentice list because it reflected what I saw in the film, even if George didn't choose to put it there.
* I like slash because queer content gets excised from so much of what we view and read and I like the fact that somebody is putting it (back) in, even if it's not going in in the same places it was taken out of :-).
* I like writing and reading slash because I like exploring the power dynamics in same sex relationships (both in fiction and irl). I sometimes see it argued that there is an absence of power games with same sex couples, and that's why slash is appealing. I don't think that's true at all. OTOH I think there's an absence of gender roles, an absence of gender-related power. So it's up to the two men (or two women) to work out power dynamics between themselves, without a set gender power dynamic to accept or reject. I think power is complex and multi-layered and changes over time and that's a lot of what I explore in my fiction. I don't think power dynamics in couples are at all limited to sex, but I think sex is a very good venue for exploring power. Who does what to whom and who decides that can be illuminating of power in the relationship and is something often explored in slash. I do, however, think that power sometimes gets confused with penetration in slash and they aren't the same thing.
* I like to write slash because it's hard to do well. This is one I never seem to see on anyone's lists, but it's a major reason for me. I think it's a real challenge as a woman to write from a male POV (particularly to write sex from a male POV) and a real challenge to take a character who appears to be heterosexual in canon and make him credibly gay and credibly himself. I love reading slash that does that, too.
* I write slash because I do think the best characters in my main fandom (X-Men) are male. I wasn't struck, at first, by the female characters. I've really come to like Jean, though, over time and do write her as a strong character. I've also introduced female OCs. I can't see writing and not writing about women, but my favorite X-Men are almost all men.
* I write slash because I want to write about being gay and about coming out while writing about the X-Men. What attracted me to X-Men in the first place was, in large part, the queer subtext. I like how the mutants can stand for gay men and lesbians in our society. I've liked the source text most when it has played with that issue. By writing about a gay and deeply closeted Cyclops, I can deal with issues surrounding coming out and internalized homophobia more directly.
* I was motivated to write in part by a feeling that a lot of the writers who wrote X-Men slash - even those who wrote well - just did not have a sense of what it really means to be gay in North American culture. I wanted to write about what it would be like for a guy who's very concerned with following rules and fitting in and who already has one conspicuous difference (those damn glasses) to have another difference, one that people won't know unless he discloses it.
* I write slash because I believe in Write What You Know. I've been actively involved in gay community all my adult life. I've heard lots of details about lots of gay men's sex lives. I've been privy to many people's coming out processes. My main character is a deeply closeted high school English teacher as well as a superhero. I haven't known a lot of superheroes, but I've sure known my share of deeply closeted high school English teachers.
* Sort of in counterpoint to the above - I write slash because m/m sex doesn't reflect my own sexual experience. I'm essentially a very private person about my sex life. I wanted to try writing about sex and had never done so before. By writing m/m sex, it's divorced enough from my own experience that it was a more comfortable place to start, in terms of not discussing my own sexual expression and not being concerned that readers might believe the sex scenes to be reflective of my own sexual expression.
* I write slash because m/m sex and desire lends itself to exploration of some themes that interest me: safety and danger, community and isolation, being true to yourself and the costs incurred, power dynamics in relationships (as mentioned above), coming out issues. It's not the only scenario to explore any of these, but it's a very fertile one for all of them.
* I write slash because I like to explore how sex affects friendship and vice versa. I think opposite sex relationships can be friendship-based, but same-sex ones more often are. I have a fairly non-romantic view of sex and I think that is easier as a reader to find fiction that depicts that POV in slash than in het (although the het stories I particularly like tend to depict friendship-based couples) and I find it more natural as a writer to express it in slash rather than in heterosexual relationships.
* I write slash because I'm a lesbian and I like turning women on. I quite like fan letters that say things like "give me more sex like that and I'm yours" even if they don't really mean it.