And the Lord went before them by day in a pillar of cloud to cause it to lead them on the way and at night in a pillar of fire to give them light, [they thus could] travel day and night. (יג:כא)
The Nesivos Shalom brings from Koheles that there are 28 crucial times in a person's life, with 14 of those times being good and 14 being bad. Just like we praise Hashem and reveal Hashem's glory through appreciation of the good times, so too, the bad times are also created for us as an opportunity to praise and glorify Hashem. The purpose of Creation is, through both day and night, to always go towards Hashem and praise Him. This must be done both in the times that are clear, as well as the times that are dark. We need the dark times also to become a whole person.
The Nesivos Shalom quotes the Zohar, that the Amud Anan B'Yom - the Clouds of Glory at day - represents Avraham. Whereas, the Amud Ha'Eish B'Laylah - The Pillar of Fire at night - represents Yitzchak. The two middos they embodied were ahava - from Avraham, and yirah - from Yitzchak. The two ways to bring out Hashem's glory is when things are clear and the weather is beautiful and sunny - This is when we serve Hashem through ahava, pure unadulterated love. Whereas when things are dark outside, uncertrainty and fear begin to creep in - This is when we must learn to serve Hashem through yirah, awe-inspiring fear. This concept of yirah means that we know there is a Judge and that there is consequences for our actions. Whereas serving Hashem with love is what motivates and inspires us through the brighter times. The Nesivos Shalom explains that there is an exception to this rule. He quotes the sefer Toras Avos, which explains that if we are able to hone in on the darker times to serve Hashem through ahava, then Hashem deals with us middah k'neged middah, and sweetens all the darker times to make them brighter.
Taking this idea one step further, R' Tzadok says the purpose of the dark times is to enable us to discover parts of ourselves that we otherwise wouldn't uncover. He discusses this by bedikas chametz by erev Pesach. We first must wait until it's dark outside and only then take a candle to find the chametz. If the light is on in the room, so everything is apparent, but it doesn't allow our eyes to catch the small cracks and crevices in the room, where our chametz is most likely hiding. Once everything is dark, and then you take your candle to illuminate every angle and perspective - we can find the hidden chametz. To discover where our yetzer hara and egos are hiding, we need to ignite our candles specifically when the times are dark. The darkness is meant to sensitize us to what we can improve most upon in our relationships with Hashem.
~ Rabbi Tendler is the Rosh Yeshiva and founder of Yeshivas Lev Zion.