When given a choice, I have never understood how it is that some people choose to be miserable.
They may not outright say "I have chosen to be miserable", and they don't have to. Their actions make that clear enough.
I have never made the conscious choice to be miserable, but I have--on occasion--taken actions that guarantee it. As a result, I have paid for it in my quality of life, my attitude, and my prospects for a better future.
When I make the conscious choice in word and deed to be a positive person, it has made a difference.
Look at your words and actions, and see how you can change them to make a difference for you and yours.