Shmuel Beis Perek 6
The Pasuk says “וירכיבו את ארון הʹ על עגלה חדשה,” that Dovid drove the ark of Hashem on a new wagon (6:3). Chazal teach us that since Dovid said in Tehillim “זמירות היו לי חוקיך” (Your laws (the Torah) were like songs to me)”, he was punished for forgetting the mitzvah that the Ark must be carried on one’s shoulders, and he transported it on a wagon which caused devastation.
The Avnei Ezel asks an obvious question on this: what is the connection between the mitzvah ofבכתף ישאו (that the Ark should be carried on one’s shoulders) and what Dovid said in Tehillim? Furthermore how is this punishment equated as measure-for-measure?
He answers that the real way to experience and live Torah is through true toil and effort; as Chazal say “the Torah only exists in someone who (metaphorically) kills himself for it”. He adds that Torah and Mitzvos which are performed with laxity and insufficient effort are not adequately fulfilled. Therefore, one must bear the weight of the Ark (the Torah) on their shoulders and not just carry it through less difficult means.
Dovid called the Torah "זמירות"; this represents his joy over it. However, this joy and elation could also denote effortlessness and ease. Consequently, Hashem caused Dovid to forget the mitzvah of בכתף ישאו of bearing the weight of the Torah on one’s shoulders due to treating the Torah with insufficient care on his lofty spiritual level.
This teaches us a valuable lesson. We must always toil and invest effort into that which we do and not try to avoid effort: being motivated is an important quality in and of itself. As we learn in Pirkei Avos, ‘לפום צערא אגרא’ – “according to the pain (investment) is the gain”.