"Since we must go through the storm before seeing the rainbow, we often must experience sorrow before joy." This does not pair up.
"As a result we must go through the storm before seeing the rainbow, we often must experience sorrow before joy." This is not an after effect sentence.
"Just as we must go through the storm before seeing the rainbow, we often must experience sorrow before joy." This sentence is both fluent and linguistically correct.
"Even though we must go through the storm before seeing the rainbow, we often must experience sorrow before joy." This statement is not disregarding anything--so the phrase "Even though..." would not make sense.
Thus, the correct answer is option C. Just as. This is because all the other options do not transition correctly into this sentence, or at least, in this context.