Tom can take pain. He can eat the shit if he has to and even enjoys it sometimes. Him and Shiv were just not a good match. Tom and Greg work because the pain they deal to each other is pain they understand. Furthermore, I think it's pain they want. Tom doesn't want distance. He wants his other person's pain to come from an idea that he might not really care, because the truth is that he does. The switch will always flip back to love with him. He likes making somebody feel scared, but he loves soothing their fears. In season 2, Rhea says to Logan before she quits, "I can't see the bottom of the pool. I don't know if you care about anything." The one thing we have always known about Tom, however, is that he cares in abundance. He cares too much for the people around him to tolerate, too much about himself, too much about what other people think. For him, the tiles of the pool are made of care and that's why we have Tom and Greg in the end together. Yes, Tom leg-shackled Greg to him without a choice and their relationship started with hurt, but in the end it would've always come to love. I don't believe in a universe where Tom made Greg shred those documents but didn't wine and dine him afterwards. Beyond character analysis, that's how WSCP as a system operates: makes you complicit in others' crimes but dulls your pain through material rewards, until you start to like committing crimes with other people too. Tom and Greg are the perfect subjects for the system to churn up-- Greg even more so because he doesn't care like Tom.