Question:How do we balance seriousness (working hard on ourselves) and laughter (based on emunah that everything is in the hands of Hashem) in our life?
That is a very good question. It is complicated but here is a brief answer: there is a verse, לאדם מערכי לב ומה' מענה לשון (“To a man belongs the arrangements of his heart”); the depth of our power of bechirah (free will) comes from the heart’s search for the truth and from the will for holiness. The rest is up to Hashem. Our entire avodah, from a deeper understanding, is to perform deeds that purify our heart which brings us to the greatest possible purity of heart we can reach. All that we can do is to choose to act correctly when we access the depth of our heart, and that is our role in bechirah. When a person truly purifies his heart, his actions will surely be the will of Hashem. But the actions we then do are not really from our own free will; they are done by Hashem, for Hashem “does” everything.
If so, being that Hashem does everything, what is the depth of our bechirah and our avodah on this world? The depth of our avodah is to purify our heart as much as we can, and to perform deeds from a purified heart. But our actual avodah is always "מערכי לב" – it is only that part which is “to man”, in our hands to do. That is entire role on this world, and it is only this aspect which we can get reward for. Although it is ultimately Hashem Who does all actions, it is still a result of our free will, therefore, we will get reward for it. Therefore, a person has to try his best, and the rest is up to Hashem.