Jack had been thinking about their situation and decided that Pitch was touch starved. Each time he initiated a new kind of contact, Pitch was wary at first, but quickly indulged in the activities as often as possible.
Now that Jack thought about it, Pitch had been starved of any human contact for centuries, and the signs started long before Jack started the touching thing.
The earliest sign he remembered was Pitch's inability to understand most of his jokes. They exchanged witty banter, but the first time Jack made a joke, referring to Bunnymund as a kangaroo, instead of laughing, Pitch nonchalantly corrected Jack.
Jack ended up having to explain all of his jokes to him, which served to make them much less funny. Eventually, he just explained the whole concept of jokes to him, and he caught on.
Jack had figured he would do most of the talking in the relationship. Pitch seemed kind of quiet and reserved, but that didn't seem to be the case. In the early stages of the relationship, Pitch talked with Jack more than expected, but once he began to trust Jack, he couldn't keep his mouth shut.
Jack didn't mind; he loved hearing Pitch's voice, and he always left plenty of room for Jack to be involved in the conversation. They talked for hours at a time, usually several hours after they had already said goodbye and just wanted to say one more thing, which lead to another, which lead to another, and so on.
It occurred to Jack that Pitch hadn't been able to say these things to for ages, literally. Some things, it almost seemed as if he'd saved the story to tell someone later. He often brought up things that happened centuries ago.
"I just remembered I wanted to tell you, one time, in 1253, there was a little boy who was afraid of grass. Grass! Can you believe it? He thought there were tiny creatures hiding in the grass that would chew on his ankles."