It's a one-way track
I ain't never coming back
He doesn't quite know where he stands, but he does know one thing. The path he is on is one way and even if he wanted to, there is no turning back. Past is the time to make the decision to stop and take a different path – a safer path. It just isn't possible any more.
He is in too deep and it comes down to one thing.
There is a reason why Weiss and some of his not so friendly co-workers have been badgering him about his behaviour and feelings toward a certain double agent. Not one person knew the extent of his attachment then. They're mostly still only making wild guesses. Sydney though knows now.
It all started when her mother of all people gave him advice to tell her daughter how he feels about her. He showed Sydney his grandfathers watch soon after and told her it stopped the day they met – and his grandfather had once said you could time your heart by it. Sydney made his heart race faster with a few words. "Me too."
Later when they were in the wreckage of SD-6, the evil enterprise finally fallen, they shared a look and then a kiss. Things escalated, leading to where he is now.
Trapped by his emotions. Tied and torn and riding a train that only ever travels forward and never back to where it had previously been. Searching new grounds. Exploring. Seeking to know more, to feel more, to be more. She deserves no less than perfect and so that is what he strives to be.
His life is so much worse with her in it: more dangerous, filled with pain and loss and torture and hope and hopelessness. It's also better. Amazing, even. And therein lies his greatest downfall and greatest achievement. Through it all, he pushes on. And he doesn't, despite the bad, ever intend on going back.