"And they shall make for Me a Mikdash, and I will dwell among them" (Shemos 25, 8). The Seforno explains, "and I will dwell among them: to accept their tefillos and their avodah." Our connection to the Shechina enables the acceptance of our tefillos and our avodas Hashem.
This idea is also outlined by the Rambam in Hilchos Teshuva (3, 7). He writes, "[Come see] how great the attribute of teshuva is. Yesterday, this person (who sinned) was separated from Hashem as the pasuk says, 'Your transgressions caused a separation between you and Your G-d.' He would call out [in prayer] but was not answered, he would perform mitzvos and they would be torn up in front of his face. However, today, he clings to the Shechina, he calls out and is answered, and he does mitzvos and they are accepted with pleasure and happiness."
Rav Wolbe (Alei Shur vol. I pg. 208) notes that it is evident from the Rambam that not only do we need our tefillos to be accepted; we also need our mitzvos to be accepted! When do we merit Hashem's acceptance of our tefillos and avodas Hashem? The answer is, as the Rambam writes, when we cling to the Shechina.
How does one reach this awesome level where he is assured that his avodah is accepted? The answer can be gleaned from what the Rambam writes at the end of Meseches Makkos. "One of the fundamental beliefs in the Torah is that when a person performs one of the 613 mitzvos properly without any foreign intentions, rather, he does it solely for its sake out of love [of Hashem], through its performance he has merited a portion in the world to come. This was what Rebbi Chananya ben Akasha intended ... since there are so many mitzvos, it is impossible that over the course of a lifetime that a person will not perform at least one mitzvah properly, and via that mitzvah he will merit to live in the next world." When one performs a mitzvah lishmah - solely for the sake of Heaven, he has connected with Hashem and his mitzvah will be accepted wholeheartedly.
This level of greatness was demanded at the time of the building of the Mishkan. The donations were to be given, "for Me," as Rashi explains they were to be given solely for My sake. Rav Wolbe (ibid. pg. 328) cites Rav Yeruchom who stated that even a holy intention such as money donated so that the Shechina should dwell among them, was not considered entirely lishmah - purely for the sake of Hashem, and was not accepted.
When one performs a mitzvah lishmah, he connects with the Shechina and he can be assured that his avodah will be accepted with pleasure and happiness! This in no way means that we are to detract from our avodah in any shape or form out of fear that we won't succeed in performing the mitzvos properly. The very opposite is true. The more mitzvos we perform, the better the chance that we will succeed in reaching this ultimate level of lishma - the very purpose of our existence.