תַּ֗חַת אֲשֶׁ֤ר לֹא־עָבַ֙דְתָּ֙ אֶת־יְהוָ֣ה אֱלֹהֶ֔יךָ בְּשִׂמְחָ֖ה וּבְט֣וּב לֵבָ֑ב מֵרֹ֖ב כֹּֽל׃
<span class="x" onmousemove="Show('perush','El Rambam explica este versículo, y el principio del siguiente, en el <b>3º Capítulo</b> de Las Leyes del Estudio de la Torá.',event);" onmouseout="Close();"> Por cuanto no serviste á SEÑOR tu Dios con alegría y con gozo de corazón, por la abundancia de todas las cosas;</span>
Ibn Ezra
an abundance of everything denotes either everything that you might desire, or everything that you need.
Or HaChaim
תחת אשר לא עבדת…בשמחה, "Because you did not serve…out of joy, etc." This punishment and what follows until verse 58 אם לא תשמר לעשות is for non-performance of the positive commandments of the Torah. The Torah repeats the same words in verse 58 to make certain that we appreciate which set of curses is in response to which set of sins. The reason the Torah speaks of our failure to perform the commandments out of joy is to contrast joyful מצה-performance with the absolute opposite, i.e. exile experienced amid the most depressing conditions. The Torah paints a picture of the punishment fitting the crime. The Jewish people who shook off the yoke of G'd's commandments will instead have to bear the far heavier burden of their captors' yoke around their necks.