If you read it- it's a deal breaker- They are very private about what they write and will only show you the things in it they want you to see. If you betray their trust- that's it.

Even so- you would love to know what's inside- You have a fascination with what they have written on the pages, especially the parts about you, but you know that some things are better left alone.

You know not to make fun of their diary- It's important to them- it works like a kind of therapy and makes them feel better at the end of the day. It may not be your cup of tea but it's important to them that you respect that it has value in their world.

Bed time revolves around this activity- They probably have a little routine of a night where they get snuggled up and write in their journal so you often have to wait for sex, a goodnight kiss, a cuddle or a conversation.

They have a record of your time together- If you ever want to know what you did on a particular day as a couple- they will be able to tell you.

It can dispel arguments- You find that if things are a bit frosty or you've had a fight- once they've written in their diary, they calm down because they have had time to process things by noting them down.

You know deep down it prevents them saying things that might hurt you- By writing down the negative feelings they have when you're going through a rough patch as a couple- you get a sense that they think twice before they speak their mind. Sometimes you wish you had a technique that helped you to do the same. 

They can highlight patterns- If they see that every night this month you've stayed in- then it shows that you're overdue for a date night and they will remind you of this fact.

They always appreciate their own successes and yours too- Because they record both the good, bad and mundane- it makes them focus on the things that go well in the life you share. Constant reminders of the positives can be uplifting for you both.

Gifts are easy- You always know what to buy them when Christmas comes around and if they happen to use a notebook- it sometimes falls on birthdays and anniversaries too- which is an added bonus.