I had the same reaction to John/Finch in Person of Interest.
On the other hand, I don't know if any fans have ever "slashed" Shawn and Gus from Psych, but if they have, I would never buy it--yet I think they are hilarious and very believable friends.
Which made me wonder, What's the difference?
Why Greg/Nick but not Nick/Warwick, even though Nick & Warwick work together more (at least initially)?
Granted, there's a subjective element here. Sure, Kirk/Spock is a given. But as mentioned above, Jesse/Steve never occurred to me until I was presented with the idea. (I was trying to find a picture of Charlie Schlatter for another post!)
Greg & Warwick are friends and like each other but gain nothing, artistically speaking, from the relationship moving forward. Greg/Nick, however, are established early-on as having a link--Greg wants to be a CSI. Nick is a nice guy who gives another guy a break. The combination of personality and desire establishes an investment that goes beyond immediate needs to something deeper, such as identity. (Maslow's Hierarchy is, in fact, raised in one of Nick's early episodes.)
Jesse and Steve is less obvious, but Jesse has a kind of Greg-like wish to emulate Steve. Steve is so entirely unattached to anyone except his father, it is easy to invest his lack of attachment with lonely need. Somebody cheer this guy up! And Jesse is very cheerful.
In sum, vulnerability plays a role here. However, I think another reason for slash--despite the obvious physical, erotic, and romantic pay-offs--is, in fact, artistic sensibility. It is easy to pour scorn on fans for slash pairings. They don't deserve the scorn. Slash keeps a relationship moving forward and growing. From an artistic point of view that forward momentum is a powerful incentive.