You have two selves. Your short-term self and your long-term self. A promise is an asset to your short-term self but a liability to your long-term self.
When you're happy (or feeling other positive emotions), you become agreeable and quick to make promises. But by making a promise, you give up ownership over your future time.
I learned this the hard way
In April, I was a guest speaker at a German high school. Everyone was thrilled by the experience. So when they asked if I would come back before the end of the school year, I said of course.
But now I noticed my time in Germany is coming to an end.
And I don't want to spend a day traveling north and sleeping in a village away from my life in Konstanz. But breaking a promise isn't a solution either. The solution is to make fewer promises to begin with.
I'm not saying avoid all commitments.
A life with zero responsibility probably suffers from no meaning. My point is to be slow to make promises. And since positive emotions can cloud your thinking, it's best to make promises without them.
If you want to own your time and be more dependable, make fewer promises when you are happy.