Sometime around the year 2000, I wrote in my journal that all problems could be solved if all the notes we wrote on scraps of paper could be collected on one big wall. We could see everything together and make sense of all combined variables. I believe we unconsciously do this, and try to recreate the attempt in our daily lives. All of the notes I write look something like:
"Trapiche Malbec, Argentina 2008"
"HW 5 due by April 29"
"Eggs, spinach, salsa... (grocery list)"
"Full Metal Panic (an anime I've been wanting to check out)"
So it looks more like a to-do list, but if you were to add all of my to-do lists together, you could see my habits. For example, I like anime, wine, eating eggs, and sticking to deadlines, as those things are typically subjects I write down. Over the years, I've been able to troubleshoot my diet, my skin, and basically all other personal problems by looking at the topics written down. There is a definite correlation between my immunity (or lack of) and the amount of bread and carby food I eat. When I eat a relatively higher diet in protein, I notice little wins like my skin clearing up, less water retention, and fast growing nails, which are all related to health. I write down all of these things, so to later see what works and what doesn't.
Physical health is also related to my mental health - was I stressed with school or barely scraping by? How were my relationships like at that time? See what I did there? I just connected health and money and relationships in an organic way. While correlations between these things sometimes aren't readily apparent, it's helpful to see all of the data as a set instead of just a select metric (e.g. weight on a scale every day).
Other observations through note writing:
Problem solving and coping with something are directly proportional to the amount of good sleep I get.
Lifting weights leans me out way better than running several miles a day.
Certain people give me energy, and certain people drain energy.
Knowing myself is invaluable for decisions about money or lifestyle.
Change is cool.
A note about change... I've been keeping journals since I was in fourth grade, although the frequency of my writing has varied throughout the years. Some journals include copious self discovery while others are merely records observations. If nothing else, I can see the change in myself and in others, and it's pretty neat (I finally found some good hair products, and a great hair stylist for example).