Having said that, it's often quite enjoyable to flick through a journal of a year or five ago and see how far you've come. Other times they serve well as books of memories if you feel the need to reminisce. The more time that has lapsed between the time of writing and the time of reading the better - you will be able to read it with less attachment and a clearer perspective. Journals contain content that is personal and at times not anything you would want to verbally share with anyone else. You would be restricting what you really needed to write.
Prevention is better than cure.
If you are living with people you love and trust let them know that your journal is a private place and could they please refrain from reading it if they see it lying around. If you still have some privacy concerns make sure you keep the journal in a safe place or take it with you wherever you go. This is not very practical so developing a trusting relationship with your partner, family or housemates is obviously the better option. If after taking all precautions someone does read it, it's not the end of the world. Writing knowing someone else is going to be reading your journal, as mentioned above, influences what you write and in turn affects any benefits you could be receiving.