As a person who has been journaling for more than twelve years ever since a very compassionate friend advised me to, I can attempt to answer why journaling seems to reduce stress.
AliceD mentioned that it's a way of writing down todos. That’s true. I also have Google Tasks for that.
The friend of mine deals a lot in algebra. We had spent a lot of time together working out mathematical problems, playing the guitar as well. It was thoroughly enjoyable to see him logically work towards the right answer. He believes in working things out by deriving from first principles.
We are quite religious so our journaling is quite spiritual in nature.
I didn't go into journaling knowing how good it would be, but looking back the benefits are invaluable.
Absolutely anything can go into your journal. I love to draw diagrams too.
In life we all make mistakes. I write down the fool things that I have done so that I can correct myself. Very often we do not know why certain people are antagonistic or get angry with us. This is because we are not omniscient like God. We do not know what the other person is thinking. By writing events down we are working out problems in our interactions with our fellow human beings. It greatly simplifies our thought processes. We can solve problems much more efficiently. We can improve our relationships with people.
I also write down what I am grateful for :
I used to have a lot of ‘noise’ in my head. I would think about what people said in great detail. Often people tell me not to think too much. It’s those stressful experiences that get me thinking the most. By writing everything down, I’m offloading my thought processes into my journal.
I sincerely want to help the person who posted this question as I feel so much affinity towards the points in Buraian’s question. I hope that my answer would also help the community and people who chance upon this question. I wasn’t expecting to write content targeted at scientific journals so please forgive my very informal first draft. I read the guidelines that personal experiences are valid answers so that’s why I decided to answer this question.
Deaver, S. P., & McAuliffe, G. (2009). Reflective visual journaling during art therapy and counselling internships: A qualitative study. Reflective Practice, 10(5), 615-632.
Smyth, J. M., Stone, A. A., Hurewitz, A., & Kaell, A. (1999). Effects of writing about stressful experiences on symptom reduction in patients with asthma or rheumatoid arthritis: A randomized trial. Jama, 281(14), 1304-1309.
Jamison, S. G. (2007). Online law school faculty perceptions of journaling as professional development: Influences, barriers and pitfalls (Doctoral dissertation, Capella University).