Writing a gratitude journal is one of the most commonly prescribed activities for mental well-being, and certainly has some pretty compelling evidence for the mood-boosting effects it delivers.
I think it is helpful to understand (at least in my own layman's terms) some of the reasons why it helps us. Our brains tend to look for threats and potential negative impacts more than they look for the positive in our environment. While this is a sensible way to protect ourselves it can lead to an unbalanced view of life. If we write down three things we're grateful for each day (and ideally they should be different every day), it alerts our brain to have a "things to be grateful for" radar running in the background all day, so that we'll have something to note down. This can balance out our threat alert and bring us to more balance and contentment.
If you haven't tried doing this simple exercise, why not give it a go and find out why it is SO popular. Keep it simple - just three quick things (don't let the word "journal" put you off!)