You're feeling really good about things, and then someone comes along and says something that really hurts. All of a sudden thought drops and everything loses its lustre, and now you are feeling bad about things.
What happened? Is there some kind of mental gravity at work here - as long as you are flapping your wings or pedalling or rowing or... expressing joy - you are fine. But something comes along to take away that joy, stops the activity, and down, down you go. Seems like it.
Many try to maintain feeling good by keeping themselves entertained, or by taking drugs of some sort - and avoid thinking too much about it. But perhaps we can take a moment now to think about it.
If we are dependent upon things outside of our control to make us happy, like other people, the weather, the stock market, just plain luck, etc., our happiness is vulnerable and our thought will drop "at the drop of a hat." This we call materialistic thinking or idolatry, because our god is not God, Spirit - no rock to build upon.
If, on the other hand, we rely on that spirit within us that acknowledges God is all our ways, our joy is from Him and cannot be shaken. As the hymn says, "When all material streams are dry, Thy fullness is the same." The only way for our thought to drop is if we turn away from Him to material things, personal sentiments, and then like Superman exposed to kryptonite, we lose our dominion.
The science of maintaining thought is expressed beautifully here: "Progress takes off human shackles. The finite must yield to the infinite. Advancing to a higher plane of action, thought rises from the material sense to the spiritual, from the scholastic to the inspirational, and from the mortal to the immortal. All things are created spiritually. Mind, not matter, is the creator. Love, the divine Principle, is the Father and Mother of the universe, including man." (S&H)