Disclaimer: I don't own anything, except an ink pen of my own.
Sam likes to write. Most of the time he's on his laptop, but every so often, when Dean is asleep, or out hustling in a bar somewhere, or in some girl's bed, Sam gets out an old notebook and an antique ink pen Dean doesn't even know he owns, and he writes.
He writes down their latest hunt. He thinks it might come in useful one day. If it's not already in Dad's journal he'll note every aspect in the greatest detail. If Dad's already come across the same thing then Sam just writes down anything new or noteworthy. Often he'll commit their actions and words to paper. It's all very factual. Sam's very pragmatic about what he puts in this notebook. One day, when this is all over, he hopes to pass on the knowledge and experience he and Dean have accumulated over the years to the next generation of hunters. He doesn't leave anything out, no matter how trivial or irrelevant it may seem. He's learnt that the smallest things can make a world of difference.
Then, when he's done, he checks the ink is dry before closing the notebook, blowing on the writing, if necessary. He puts it back in his duffel and pulls out another book.
To outward appearances it's just another heavy reference tome, one he knows Dean would never look at twice. Inside the covers though, it tells a different story. The plain pages are filled with Sam's neat script. This is the book he'll take to his grave with him. This is where he records his feelings and innermost thoughts. He'll never tell Dean about this book. He doesn't like keeping things from his brother, but knowing how he feels about 'chick flick moments', Sam reckons this is something to keep to himself.
Sam enjoys the sensation of the ink flowing smoothly from the nib of the pen on to the paper. He imagines his soul flowing through his fingers and into the words staring up at him. He feels more connected when he's using his pen. He thinks is more personal, more him.
If he writes for too long, his fingers start to ache and cramp. When that happens, he puts the pen down and looks at what he's written, flexing those long digits of his. He cringes at the thought of Dean ever finding this book and then smiles. He wonders if his brother has anything similar.
No, Sam doesn't like keeping secrets from Dean, but there are some things he needs to have for himself.