I don't keep a journal for a couple reasons. First, it takes time and energy.
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Secondly, I think the people in my life that care about what I would put in a journal already know what I would put there, or they can ask me.
On some level, it takes a certain amount of conceit to write in a public journal. A public journal states that you believe what you write about is important or worthy for others to read. I think that applies to all writing, though, not just journaling.
Any public journal I kept would be more of an opinion board... something to share my viewpoints with the world. I have no interest in keeping one for my own thoughts and introspections because I can use my brain to think about those; I don't need to write them down, nor do I want or need others reading them.
Recently I have been asked by several people why I don't keep a journal, so perhaps in the future I may appease the teeming masses and write some thoughts down. I have a strong feeling it will just end up reading like an opinion section in the newspaper, though.