If a painter buys some materials and applies his skill to them, creating a beautiful landscape painting, he must have a reason. He doesn't look at the painting and the spent materials, and feel the weariness in his body, and think "why did that just happen"? He starts out with a reason. Perhaps the painter is working on commission. Perhaps he sees a beautiful scene and feels a need to capture it. Perhaps he simply loves the act of painting. Whatever the reason, there is a reason there.
What is our final cause? Our reason for being? For as much effort as we put into this question, for all the uncertainty we feel about it, there is a very simple answer.
Our material cause is Christ - one in being with God the creator.
Our efficient cause is Christ, the Word.
Our formal cause is Christ, the Body and Blood that becomes part of us.
None of these causes are quite earthly, so we cannot expect the final cause to be earthly either. While we are meant to be here, each to his or her purpose, the final cause is not something finite and earthly. Our final cause must be Christ.
We are made by Him and formed by Him, to join with Him in Heaven forever. Our final cause - our purpose for being - is Christ.
You made us for Yourself, and our heart is restless until it rests in You. - St. Augustine, Confessions I, i, 1.