The Sm”S writes that this idea helps answer a question raised by his father, the Sochotchover. The Sochotchover assumed that there is no din of simchas Yom Tov on the night a Yom Tov starts because simcha is elicited by the performance of the mitzvos of the chag. It’s only after Pesach has started that we can eat matzah; it’s only after Sukkos has started that we can eat in the sukkah -- the simcha kicks in the next day, after we have had a chance to do the mitzvos. But what about on Shavuos – there are no mitzvos to do; nothing is missing when Yom Tov starts, so why does the din of simcha not apply immediately? Shem m’Shmuel answers that there is something we need to do: “l’natra dachya ila’ah” is an active, eager waiting, not a passive lack of activity.
If you want to see the difference then take note on your way to work as you pass by kids waiting at their bus stop for the schoolbus to arrive on a spring day. The kids will be joking with each other, talking, eating, etc.; in short, they will be doing anything and everything short of looking down the road to see if the bus is close. As far as they are concerned, the further away the bus is the better! Now take a look at adults waiting by the bus stop or subway stop on their way to work. Their necks crane every 30 seconds as they peer anxiously down the road or down the track to see when the next bus or train is coming, they tap their feet and shuffle around anxiously, they listen to every traffic report or announcement with trepidation lest there be some unanticipated delay. That’s the “natra” the Zohar is talking about.
The Maharal writes on the pasuk “U’Moshe alah el ha’Elokim va’yikra eilav Hashem min ha’har leimor…” that Moshe had to make the first move and ascend the mountain before Hashem came down to speak with him. Kabbalas haTorah starts by our demonstrating the desire to receive Torah; one cannot wait passively for it to drop in one’s lap.