For a while, I was going through King Solomon's view of realizing how pointless life is. I was struggling with that for a couple of weeks. But I am doing better now. Just really meditated on it and what Solomon said and a realization of life in itself. It is kind of interesting but always weird. Really thinking of the concept that you're nothing and it's all vanity (or vapor). I like it, but it made me really think about what I'm accomplishing this in life and why.
I was thinking that when I die, it all means nothing and will soon be forgotten. I do remember why I am here and what I love, but I was thinking I have taught many people and it's a repeating, never-ending process. I've seen people come to Christ, and it's never-ending, which is a good thing. But what I mean is, it's always coming to the realization this is the only thing you can take to heaven with you. That's where your treasure is for Christians, which in my opinion is the reward, besides being with God, of course, is seeing all those people you help bring to Christ and knowing they are there with you. The joy is seeing the fruit of your labor and how awesome to see all the people's lives you effected and who made it to heaven. In a way, the most valuable treasure, human life, is what you can bring with you in a way. It's each person's choice to follow God but at the same time to have part of it and know it can last for eternity is always awesome.
"According to the grace of God which was given to me, as a wise master builder I have laid the foundation, and another builds on it. But let each one take heed how he builds on it. For no other foundation can anyone lay than that which is laid, which is Jesus Christ. Now if anyone builds on this foundation with gold, silver, precious stones, wood, hay, straw, each one's work will become clear; for the Day will declare it, because it will be revealed by fire; and the fire will test each one's work, of what sort it is. If anyone's work which he has built on it endures, he will receive a reward. If anyone's work is burned, he will suffer loss; but he himself will be saved, yet so as through fire" (I Corinthians 3:10-15).
It seems to me that you've discovered what Paul talked about in I Corinthians 3:10-15. Paul saw himself as a master builder. Paul's gift from God was to lay the foundation of the church (Ephesians 2:20). His favorite work was to go where others had not been to get the cause of Christ started (Romans 15:20). But in even laying the foundation well, Paul knew that it would take other laborers' efforts to build upon what Paul had begun. The foundation is critical, but it isn't the whole building.
Seeing each Christian as the building material being used in this effort, Paul acknowledges what is placed is not all the same quality. Becoming a Christian is not a guarantee of salvation. Only those who endure to the end will be saved (Hebrews 3:5-6; 4:11; Revelation 2:10). We often focus on the loss of a Christian who is not able to endure to the end, but here Paul mentions the impact such loss has on the teacher. It is a joy to see the efforts of your work last. It is a sorrow to see them come to naught. In either case, the preacher is saved because he labored for the Lord, but the hardship he faces over those he labors can be vastly different (Ezekiel 3:17-21; 33:1-9). "For what is our hope, or joy, or crown of rejoicing? Is it not even you in the presence of our Lord Jesus Christ at His coming? For you are our glory and joy" (I Thessalonians 2:19-20).
If you study Ecclesiastes and come away with the impression that all of life is vain, you missed Solomon's point. What is vain are the things people typically think are important in life: pleasure, work, education, fame, authority, etc. Solomon takes each of those valued items and shows just how worthless they truly are. Yet throughout the book, Solomon points out what really matters and in the end, he gives the final conclusion: "Let us hear the conclusion of the whole matter: Fear God and keep His commandments, for this is man's all. For God will bring every work into judgment, including every secret thing, whether good or evil" (Ecclesiastes 12:13-14). When teaching Ecclesiastes I challenge people to highlight the verses where Solomon says that this matters. You end up highlighting quite a bit of the book, but people are so caught up in learning that what they thought was important doesn't matter that they forget to see the real conclusion.