The Avodah of Being B’Simcha

הַעִידֹתִי בָכֶם הַיּוֹם וגו' הַחַיִּים וְהַמָוֶת נָתַתִי לְפָנֶיךָ וגו' וּבָחַרְתָ בַּחַיִּים לְמַעַן תִחְיֶה אַתָה וְזַרְעֶךְ (ל, יט)

I testify before you today... life and death I have given before you... And choose life so that you shall live...and your offspring. (30:19)

The Avodah of Being B'Simcha:

The Imrei Chaim of Vizhnitz zy"a says that serving Hashem with joy and enthusiasm is called "chaim" because it makes a person lively and happy. However, if a person serves Hashem without joy and enthusiasm, the opposite is true.

The difference between serving Hashem with joy and serving Him without joy is that when one serves Hashem with simcha, he passes down his tradition to future generations, who gladly will follow in his ways. On the other hand, when one serves Hashem with no joy, his children won't want to emulate him and they won't follow in his way. This is hinted to in this pasuk. It says to choose "life", i.e., a life of serving Hashem with simcha, so that you and your offspring shall live, as this type of avodah will be passed on to future generations.

כִּי קָרוֹב אֵלֶיךָ הַדָּבָר וגו' בְּפִיךְ וּבִלְבָבְךָ לַעֲשׂתּוֹ (ל, יד)

For the thing is close to you... it is in your mouths and hearts to do it. (30:14)