Be a player. Be an actor and a singer. Act as if you already feel it. Don't wait for the feeling to motivate you. It can be a long wait.
American philosopher William James put it very clearly: “We do not sing because we are happy, we are happy because we sing.” Most of us believe that emotions, such as happiness, come first. Then we do whatever we do, in response to that emotion. Not so, James insisted. Emotions arise along with actions. So if you want to be enthusiastic, you can get there by acting as if you are already enthusiastic. Sometimes it takes a minute. Sometimes it jumps around. But it always works if you stick with it, no matter how silly you are doing it.
Feel silly. If you want to be happy, find the happiest song you know and sing it. It works. Not always the first few times, but if you keep at it, it works. Just fake it till you make it. Soon your happy song will show you how much control you have over your own emotions.
Zen monks do "laughing meditation" where they all gather in a circle and prepare to laugh. At a certain hour the master strikes a gong, and all the monks start laughing. They have to laugh, whether they like it or not. But after a while the laughter becomes contagious. Soon all the monks are laughing sincerely and wholeheartedly.
Kids do this too. They start giggling for no reason (often at the dinner table or some other forbidden place and the giggling itself makes them laugh). The truth is this: Laughter itself can make you laugh. The secret to happiness is hidden in that last sentence. But adults are not always comfortable with this. Adults want kids to have a reason to laugh. Since I used to drive my kids long distances to visit relatives, I was most annoyed when they started laughing and giggling in the back seat for no reason. I developed a backstroke to stifle the laughter. “Why are you laughing?” I would yell. “You have no reason to laugh! This is a dangerous highway and I’m trying to drive here!”
But adults, like me, might want to reclaim an appreciation for joyful spontaneity. We might want to confront the question, “What is the one thing that makes me most want to sing?” And then know the answer: “Singing.” What makes you most excited to dance? Dancing. The next time you ask someone to dance, and they say, “I don’t feel like dancing,” you might reply, “That’s because you don’t dance.”