It took Tom a little while to understand what was going on, to understand what exactly made Harry so interesting to him.
Of course, Tom could tell there was something off about Harry. He wasn’t oblivious, it just was that spending so long alone in the diary had dulled his senses in a certain way.
Harry was such a sympathetic ear, always willing to listen to whatever Tom had to say. Always willing to tell Tom whatever Tom wanted to know.
It almost made it too easy for Tom to bend Harry to his will. To make Harry explain what had happened the night that made him The Boy Who Lived. Tom wasn’t really in the mood to talk much after finding out Harry technically killed the older version of him. But he couldn’t abandon the plan. Since the older version of himself was out of the picture, Tom (despite now being still stuck in the diary) would take his place.
Of course, since it seemed the universe was out to get him. As soon as Tom had made that plan, Harry revealed he was a Parselmouth.
Now, normally that wouldn’t have affected Tom. So what if Harry was a Parselmouth too, Tom could adapt. But that wasn’t the real problem.
The real problem is that once Harry revealed his abilities, all of the pieces started coming together.
Tom liked to think he had a clear view of what was going on outside the diary but it was beginning to get clear that Harry was hiding things. Or things were being hidden from him.
Tom had learned what happened the night Harry became The Boy Who Lived, or at least, learned what Harry knew. And after Harry explained what happened that night, he explained what had happened after that. How he grew up and as Harry explained, a pit grew in Tom’s stomach.
Finding out how exactly the Dursley’s treated Harry was enough to make Tom put his ideas of killing Harry on hold. Tom was still upset to admit even to himself that the idea of another child being treated poorly by his caretakers still struck a chord in him despite it being 50 years later.
It was just a small moment where Tom let his emotions rule over him. So he let Harry live a little longer, learned more about how the world changed while he was in the diary.
It wasn’t till the school year was almost over when Tom realized that he hadn’t even released the Basilisk.
But only a part of Tom cared that it didn’t happen. Releasing the Basilisk would have caused too many unnecessary problems to deal with. Dealing with Harry Potter was a big enough problem as it was.
He was too self-sacrificing, too willing to talk about what he had experienced to Tom, too naïve, and that had to change.
The world was only going to chew him up and spit him back out again. And the only person who could protect Harry it seemed was Tom.
When Harry went back “home” to the Dursley’s, he brought Tom (the diary) back with him.
And for the first time in Harry’s life you could say, he had a friend while being with the Dursley’s.