It's overdue, Danny knows that. Like a lot of things in his life right now, he's put it off for too long. He's made half-hearted attempts to apologize but nothing that has made him properly face up to what he did and admit he was wrong to do it.
As much as he's used to people thinking he's cold, mean and hating him, it's not something that naturally sits well with Danny, even when he finds himself doing or saying things that perpetuate that perception of him. Over time he's learnt to ignore the desire to fix it, come to believe that it's out of his control. Inevitable. What's the point of trying to be a better version of himself if the end result is the same every time? People leave. All the more reason to give them absolutely no cause to stay in the first place.
When he knocks on Kevin's door, he's not trying to improve himself. He's going back to basics instead, starting at the bottom. Just trying to be a decent guy.
"Hey...Kevin? Are you in?" He knocks again, wondering if he's calling too late. "It's Danny. Castellano. Subway guy."