There’s actually a few reasons he hasn’t shared the journal just yet in the actual Brothers Apart storyline! (In Brothers Lost and Brothers Consulted, Dean has, on more than one occasion, stolen Sam’s journal with varying results).

In the beginning, Sam didn’t know if Dean was the brother he remembered. It was well over a decade since they’d seen each other last, so he took it slow. The journal was one of the things he kept private, and Dean respected that.

Later on, as their bond came back, and they grew to be a team, Sam discovered that without something to magnify his journal, Dean can’t read it. The tiny handwriting in it is too small for Dean to read, and it would feel plain awkward for Dean to sit there with a magnifying glass the entire time they’re looking through it.

If Sam ever gets a chance with Dean at his scale, he’d love to share it with his big brother…