ליקוטי הלכות - הלכות נטילת ידים - הלכה ג:ה
Only through ingenuousness and simplicity and a perfect and complete faith is one worthy to ascend upwards. This is an aspect of the mind and wisdom. And, [through simplicity] one merits to receive from there a great mind that is capable of thinking of original Torah thoughts, able to reveal Hashem's Kingship, and complete knowledge.
Rabbeinu stressed time and time again the importance of serving Hashem with simplicity. By 'simple' he doesn't mean poorly or without effort. When one does things in a sophisticated matter he may forget about the whole reason why he's doing a mitzvah, or he may even come to, chas ve'shalom, un-sound conclusions about the ikar of the mitzvah. The whole point of doing mitzvos is to accept Hashem's sovereignty.
In Likkutei Eitzos, Reb Nosson points out that (אוצר היראה: תמימות, אות א),
The whole point of accepting the yoke of shamayim is in order to push us to cast off and nullify all our sophistication (written as wisdom), and to only walk (fulfill Torah) with simplicty. This is because only the holy Torah is the true wisdom, and the rest of the "wisdoms" only serve to nullify Her.
Every once in awhile it's good for us to be reminded about the necessity of simplicity :)