This song is the song of your heart. If you follow its influence, it will lead you to understand more about God and act more like him, which will in turn lead you to greater joy and happiness.
The Bible contains records of several occasions where King David danced. This wasn't merely a celebration. David knew the Lord's influence within him, and he was adept at tuning his whole being into that music. One one occasion:
14 ...David danced before the Lord with all his might...
16 And as the ark of the Lord came into the city of David, Michal Saul’s daughter looked through a window, and saw king David leaping and dancing before the Lord; and she despised him in her heart. (2 Samuel 6)
Why did David dance? And why did David's dancing cause his wife to despise him? These questions matter.
Hearing and dancing to the music in your heart--the music from God's light--is not easy. It certainly isn't the default state. It isn't the most obvious path to joy. It happens despite the world. It happens in opposition to what the world calls happiness.
Life hits you right in the music. It cuts the volume. It takes out the melody and leaves you with a lonely baseline. You have to fight back. You have to learn to listen to the music. You have to go out of your way to get to quiet places to hear it. You are not designed to be defined by your life. You are designed to define your life by following the music within you, the song of your heart.
Eventually, through seeking it and living by it, you will increase the volume of your song sufficiently that you can broadcast it out into the world, the positive version of the inconsiderate people in public places that blare their awful music as if the world were created just for them.
Anyone can be happy on a sunny day. Your objective is to tune your ear to hear God's music so well that you can not only dance on a rainy day, but project light into the darkest night so others can dance, too.
When you dance to the song of the light in your heart, you will look like a fool to some. Others will resent you. Some will hate you. If you persist in dancing, some will even try to kill you. Why? Because your living according to God's song in you is living proof that they could do the same, but chose not to. You show them what it is possible to be, and they will hate you for it.
Just keep dancing. Dance even harder. Drown out the noise of everything else in this world, and dance to the song of God's light within you. If you lose your friends, dance anyway. If your spouse resents you, dance anyway. If your work colleagues hate you, dance anyway. Dance in poverty and dance in plenty. Dance in sunshine and in rain. Dance with all your heart, even if no one else hears the music.
Who you are, deep down inside, is not the rut of life that your environment has produced. It is what your song is. This is the light God has given you. It expands and develops as you heed it. Transform what is outside by listening to what is on the inside. Don't let the world act on you, you act on the world.
The song in your heart can be temporarily drowned out with busyness, drugs, alcohol, or any number of other things. No matter what you do, it will always whisper back to you in the quiet moments: this is not right, you were meant for more than this, others need you. It will never cut you a break, it will never let you make excuses for yourself. It will point out your self-deceit and your rationalization.
Find situations and people that resonate with your inner song. When light interacts with other light that has the same wavelength and phase (called coherent light), it amplifies in strength (this is how a laser works). Alternatively, light that interacts with other light that has a different wavelength or phase is attenuated--the waves cancel each other out. Find people and situations that add to and amplify the song in your heart, like a cellphone signal booster. Avoid people and situations that tune out, diminish, or obstruct the song in your heart.
For example, I have found value in certain scenes in certain movies. The themes of sacrificing everything for a greater cause, of standing up for what is right in the face of opposition, of discovering latent capabilities that were dormant until unusual circumstances were encountered, etc., have caused my song to resonate within me.
I remember an experience while watching "Wonderwoman" in the theater. I asked God why I feel the way I do when scenes like the one where she walks out of the trench and towards the enemy line happen in movies. The Lord told me that the desire I was feeling was a righteous one, and the resonating experience in these movies was good, but I should note that the fulfillment is much different because his kingdom is not of the world. He showed me that the real fulfillment of that impulse was in doing what was right in private sacrifice: in stopping what you are doing--no matter how important--to consider the needs and wants of those around you as if they were your own, in standing up for truth even though it is unpopular, in telling people what they need to hear even if they don't want to hear it, in forgiving those who have offended you seventy times seven times, in suffering for those who will never notice or appreciate it, in giving to those who have no ability to pay back.
Unlike in the movies, these actions can be taken by completely normal people on a consistent basis. You don't need superpowers or special events, you just need to love God and go about seeking to reflect his goodness in you daily walk.
These actions are not typically outward, extreme one-off actions, like charging across no-man's land or killing the bad guy. Instead, they are more like an inchworm's next tiny slug forward. God's work tends to happen as a very normal person stares into the darkness and decides to take a single step forward in spite of it all. It is one of the great contradictions of reality that the everything good and grand in creation is the result of an infinite number of single small steps forward into the darkness by otherwise normal people. It is a soul-crushing contradiction. You step forward, you regain your breath from the shock of greater realization of sorrow and evil, and you decide anew whether to do it again.
There is a popular idea that when you die you sit through a flashback of your life. This isn't exactly correct. What you have is a complete, constant awareness of your life: everything you did and everything you didn't do. Our situation will be much different than we imagine it. In this life, we have a fantastic ability to lie to ourselves about why we do what we do and how we feel about it. There, we will know why we really did what we did, and why we didn't do what we didn't do. We will lack the ability to cover our failings, fears, unbelief, and selfishness with lies. We will be stripped naked of our excuses, and we will see ourselves as we are seen by God: in truth.
In that day, we will feel joy to the extent that we reconciled ourselves to the song in our hearts, and indescribable pain and regret to the extent that we did not. This life is the time we have been given to learn to listen to that song inside of us. This life is the time we have been given to live in a way that amplifies its volume. This life is the time we have been given to lose ourselves in dancing to the song of God's light. In doing so, you will find yourself in the person you become. Like a caterpillar transforming into a butterfly, become the reason you were born.