The source of all happiness is to live in the present moment.
When you’re not honest, or even withhold something in your mind, you create a second thought process, a second thread in your head, that then has to stay active, keeping track of what you’ve said versus what you’re really thinking. That takes you out of the moment and brings you unhappiness over time. You may not realise it in the moment, but it will create stress and distraction.
So if you really want to be happy, you have to be present. And one of the core tenets of being present is to be completely honest at all times.