וצפית אתו זהב טהור מבית ומחוץ
Overlay it [the ark] with pure gold, overlay it inside and out. -Exodus 25:11
If no one was going to see the inside of the ark, why have the inside filled with gold as well? Why not just have the outside be filled with gold and fill the inside with a different metal or nothing at all? And while we're at it, why isn't the entire ark made of gold? Why is it made of acacia wood (Exodus 25:10)?
According to the Talmud (Yoma 72b), much like with the gold coverings on the outside and the inside of the ark, the Torah scholar must be genuine to the point where his interior matches his exterior. This consistency goes beyond the Torah scholar. Rabbi Gamilel also points out (Berachot 28a) that one could not even enter a study hall until one was the same person on the outside as he was on the inside. For R. Joseph Hertz, this gold overlaying meant that one needed to be pure in mind and heart as he was in outward manner.
What this commentary is meant to say is that "going through the motions" only does so much, especially if you do not allow for it to transform your mind or heart. The reason for that is that the externalities and the rituals become the ends of spiritual practice, not the means in which you transform yourself. As a result, the primary message is lost and individuals succumb to a downward spiral of hypocritical, "holier than thou" behavior in which they justify their depravity or immorality by invoking G-d's name. We all know those people who act nice to your face but are all the while stabbing you in the back, and this is exactly the kind of behavior we are to avoid.
This is not to say that we don't need to have a sense of obligation when we're having a bad day or "we truly do not feel like it." However, we are supposed to strive for something more than halachic minima. We are to strive for spiritual refinement and do so in a genuine manner that reflects our love of G-d. Take a look at Maimonides' Eight Levels of Tzedakah. Giving unwillingly, regardless of the amount is at the bottom of the hierarchy. Yes, it counts as doing a mitzvah, but we aspire for higher levels of holiness and character refinement that is illustrated by one's spiritual authenticity.
Gold is shiny, malleable, valuable, and is considered to be a pure metal. Humans do not have that level of purity, and neither does המשכן, which is why the entire ark is not made of gold. According to R. Yaacov Haber, the acacia wood is to represent our humanness. Our core is not golden, i.e., we're not angels. Our core is something much more natural and organic. עץ חיים is the Tree of Life. Trees are able to grow, and so are humans. We cannot obtain that sincerity, purity, and humility overnight. There are points in life where we might have it one day and lose it the next. To err is to be human. At the end of the day, G-d wants us to be authentic in our spirituality and go for the gold.