So, this poor chump is caught up in one of the messiest love triangles in Western literature and it feels like absolute "torture" (3). Not only has his mistress been hooking up with his BFF on the side, she treats him and his buddy like a couple of pathetic love "slave[s]" (3-4).
Here's the interesting part, Shmoopster. According to our speaker, it's super-painful for him to see his friend being treated this way and that's why he's taking the time out of his busy schedule to chew out his mistress (3-4). And by "chew out," we mean the speaker curses her (1), accuses her of getting off on hurting people (3-4), and cracks a bunch of dirty jokes about her anatomy (2, 13-14). Hmm. This mistress is either a really horrible person, or our speaker has got a real problem with women. Why else would he place all the blame on his mistress when his buddy ran out and started hooking up with her?
Obviously, male friendship is super-important to our speaker. He makes it pretty clear he loves his "sweet'st friend" (4) a heck of a lot more than his girlfriend. At one point, he calls his buddy his "next self" (6), which tells us that he really identifies with the guy and feels closer to him than anyone. He reinforces this idea by talking as if he can literally feel the other guy's pain (1-2) and by going on about how his friend is so close to his "heart"(10-11, 14). At one point, he even offers to sacrifice himself to the cruel mistress just so she'll stop hurting his pal (9-10). Gee. How generous.
Think about it, though. If his mistress actually took him up on his offer, here's what would happen:
(1) He'd get to have his BFF all to himself and not have to share him with anyone.
(2) He'd get to keep sleeping with his mistress.
Wow. This guy is so generous. We should all be lucky enough to have a friend like that.
We know what you're thinking. If the speaker is only concerned about his friend getting hurt, why does he spend the entire sonnet talking about his own feelings? You're right. That's why we went ahead and counted the number of personal pronouns ("me," "myself," "I," "my") this dude uses to describe the three-way relationship. (We counted 17 but go ahead and double-check that number if you want.)
Our guess is that he feels like the odd man out in this freaky love triangle and he doesn't like it one bit. You did notice how he says he feels like he's been abandoned ("forsaken") by everyone and that it feels worse than being crucified, didn't you (7-8)? If not, then go to "Symbols, Imagery, and Wordplay" and we'll tell you all about it.