From Singing in the World (#1) by Rabbi Avigdor Miller
“Amar Rabbi Yehoshua ben Levi kol ha’osek b’shira b’olam hazeh, zoche l’omra l’olam habah — If a man is busy saying song in this world [osek means he is busy, and b’shira means saying song, not prose] he’ll be rewarded that he’ll continue to say this song in the next world” (Sanhedrin).
If a man says things are good, that’s prose. But suppose you get a call that you won the lottery, you won’t speak prose, you will speak in poetry, you will speak in lyric words. You might even dance too.
If a man goes around in this world unhappy — he says This World is nothing, this world is just darkness and bitterness, it’s only we’re waiting for the real World to Come — that man will never understand his purpose in this world.
You have to be busy singing in this world, and thanking and expressing gratitude
“Kol ha’osek b’shira,” you have to be busy singing in this world, and thanking and expressing gratitude. And if you do, what’s going to happen? “Zoche l’omra l’olam habah,” he’ll be rewarded that he’ll continue to say this song in the Next World. If you eat the knishes properly in this world (the knishes in this world that I’m talking about are a different kind, they are better than the one that is served by the caterers), then that’s the preparation for the great happiness that’ll be served in the World to Come.
The Gemara in Sanhedrin quotes a pasuk, “Shene’emar ashrei yoshvei baisecha, od y’hallelucha selah — as it says, ‘happy are those that sit in Your house, Hashem, still more will they praise you forever’” (Ashrei). The next time you say Ashrei, (you do three times a day!), think what you’re saying. That’s the whole program in this world. Happy are those who sit in Your house, still more will they praise You forever. By sitting in the house of Hashem, we’re practicing up for the great song that we shall sing in the World to Come.