Why do you think Maniac wants Grayson to talk about his past? Why do you think Grayson finally agrees to tell his story?

The text doesn't flat out say why Maniac is so interested in Grayson's past, so feel free to state what you think and support it. I think in general Maniac wants to hear about Grayson's past because Maniac is a nice, caring kid that likes to learn about other people. As Maniac learns about them, he finds ways to invest in them and help them. Grayson isn't the only character that Maniac does this with either. Piper, Russell, and Mars Bar all benefit from Maniac's investment in them. Maniac seems to naturally want to help people, and he can't help Grayson if Maniac doesn't know anything about him.
At the end of chapter 24, Maniac asks Grayson for a bedtime story, and Grayson claims that he doesn't know any stories. Maniac doesn't believe it and asks Grayson what he wanted to be when he grew up. Grayson answers, and that gives Maniac a little doorway into Grayson's past. The next chapter has Maniac pestering Grayson about his past baseball career.

After a story like that, Maniac couldn't just stay behind, so he tagged along when Grayson went back to work. He helped the old man raise a new fence around the children's petting farmyard.

I believe that Maniac is truly interested in the stories. I think Grayson initially tells about his past in order to appease Maniac or get Maniac to stop bugging him about it. I also think that Grayson is finally happy to be able to tell his stories to somebody that genuinely cares about the stories and him. It feels good for him to talk about the past.