Beshrew that heart that makes my heart to groan For that deep wound it gives my friend and me. (1-2)
It's obvious from the opening lines that we've got a love triangle between our speaker, his mistress, and his "friend." But, the speaker never blames his friend for his actions in Sonnet 133. Instead, accuses his mistress of breaking both men's hearts, or giving them a "deep wound." Notice how our speaker only describes one love wound here? It's as if the two men are so close that they share the exact same pain. Basically, he's saying something like "Hey, girl, you've hurt my friend and that hurts me."
Is't not enough to torture me alone, But slave to slavery my sweet'st friend must be? (3-4)
By now, it seems pretty clear that our speaker's number one priority is to stick up for his buddy, who's been turned into a pathetic love slave by the mistress. We've just got one question: how can his friend be the "sweet'st" when the guy has gone out and hooked up with the speaker's girlfriend? Are we supposed to believe the mistress has seduced some innocent guy who shouldn't have to accept responsibility for his actions?
Me from myself thy cruel eye hath taken, And my next self thou harder hast engrossed. (5-6)
When our speaker calls his buddy his "next self," we really get a sense of how much he identifies with his friend. By the way, Shakespeare's giving a shout-out to the proverb "a friend is one's second self," which was a pretty common idea that dates back to Aristotle and Cicero. In fact, Shakespeare uses it all over the sonnets, especially #42, where the speaker says "my friend and I are one" (13), and then tries to rationalize why it's okay that the guy has been hooking up with his mistress.
Prison my heart in thy steel bosom's ward, But then my friend's heart let my poor heart bail; Whoe'er keeps me, let my heart be his guard; Thou canst not then use rigor in my jail. (9-12)
This is where our speaker offers to sacrifice himself to his cruel mistress so that she'll stop torturing his friend. He says something like, "Go ahead and keep my heart in your heart's jail, but please let me 'guard' my friend's heart." What's odd is that he imagines his own heart as a jail within a jail, which means his friend is his prisoner. In other words, the speaker's fantasy is to keep his friend closer to him than the woman involved in the love triangle.