When the couple got into the car, Cain watched it pull out and leave before he started his own engine, and rode after them. Coincidentally, he caught up to them at some lights, and eventually was separated by a couple of cars because he wasn't zipping ahead through traffic like he used to. He'd calmed down in a lot of different ways throughout his life, when he'd decided to settle down. He'd thought David would be a part of that, but it looked like he'd well and truly made his choice, because he was going home with Bill.
Then the car Cain recognised as theirs turned up a different street and he could no longer call it coincidence that he was turning after them. He wasn't planning on doing anything, certainly not, but he was curious about Bill's living arrangements. Also, after David dropped Bill off, Cain would have the chance to talk to David privately. He wanted to explain he hadn't said anything to Bill, he'd respected David's wishes, that there were no hard feelings, that he was still...
a fool. Here was a moment that he could've taken a right instead of continuing to follow the car, heading towards his own place. He didn't follow through, because there was still hope. All those feelings that had been summoned upon seeing David again, of being rescued by him - regardless of Bill's presence - he still felt bouyed by them. He wanted to speak to David unhindered, to ask him how things were going (especially after witnessing an argument), and maybe find out that David had regretted his choice and felt stuck with it. Cain wanted David to know that he was available. Foolish or not, it was how he felt.
So he followed them, expecting the car to stop somewhere and for Bill to get out. He certainly hadn't considered the pair were living together.