I suggest this based on my own life. I've had moments where I was fundamentally unhappy, and despite having a partner and friends on the books, I was effectively alone. Likewise, I've had moments where I was happy but alone, but didn't struggle to build up meaningful connections relatively quickly.
The reason I suggest the feedback loop mechanism isn't because writing in a journal is going to directly bring you happiness anymore than reading a book on finances will make you rich. It's because it gives you the insight into your life so you can make the changes needed.
As a side note, if you want to find friends and companions, you need to engage with others and have shared activities and rituals. Check out meetup, check out your local social groups (Elks, etc). Attend events, talk with other people, etc.
If it's not clear what I'm saying I'm saying that just writing in a journal everyday and someone will be happy is not enough to make someone happy. They do actually need love and friends and physical contact and purpose.