A diary is a book where we keep our secret thoughts. However these secrets are usually cherished by the girls who write them into their diary. Of course men are just as likely to keep these kinds of records but the books that they write them into are usually called journals rather than diaries. Whether for better or for worse, the word diary has taken on a feminine connotation in recent years that it cannot be easily separated from, but this does not necessarily carry over into the dream world.
If you have a dream about a diary then this means that you are thinking closely about your history. You are thinking closely about the past and you have probably been reflecting quite a bit on some of the decisions you've made in the past that got you where you are today. This is what happens when you dream of a diary; you go out there and try to recapture your glory days with your kids.
A diary is a place where you guard your secrets but these are often the most childish secrets and sometimes it is best to simply let them go. If you are having a dream in which you are reading your own diary intently then this symbolizes the idea that you don't really know yourself all that well. You're reading your own autobiographical document to get a good idea of yourself. This is a bad thing because it means you are not being honest with yourself about your feelings. Strive to be more open to even yourself about how you feel on a certain issue.