In the Old Testament, there is a famous story about Joseph's dreams (Gen 37:1-11).
He had dreams about himself.
All the family members bow down to him. Even the sun, moon, and the stars bow down to him.
The focus of his dreams is self-aggrandizement: To be the center of the family and the universe.
His dreams are so self-focused a dream.
Just to be a great person is not good dreams. They are naive and hollow.
Don't preach to others, saying something like "be ambitious."
Ambition without a good purpose of life may be dangerous.
One's dream must be for self and others as well.
Because of his naive and self-centered dreams, Joseph was hated by his brothers.
One day, he was sold into slavery and taken to Egypt.
There he came to realize that he was nobody, getting through all kinds of ordeal and difficulties.
In Egypt, he probably learned new life lessons that he was nothing.
He realized his dream was wrong.
He needed God at the center of his life.
Don't read the mustard seed parable through the lens of Joseph's dreams. This parable is not about ambition or success. Sometimes people misunderstand this parable and preach something like this: "You can become whatever you want. You can be the most successful person." If that is the meaning of this parable, Jesus would have used a cedar tree as a metaphor for God's kingdom. But Jesus used the mustard seed and mustard plant as a metaphor for God's rule. The mustard plant is not a tree and grows several meters high only, incomparable to the size of a cedar tree.
The point of the mustard parable is a small seed should not be ignored. It has potential and grows enough. Mustard plants are very useful to both birds that are sitting on the branches and people who use them as food or medicine. Mustard plants are not found in lofty deep mountains where cedar trees live. They are everywhere nearby the village, just like the good news of God spread like wildfire to everyday people in their neighborhoods. While cedar trees may be used for rich houses or a king's palace, mustard plants are very useful to everyday people.
Small things or small beginnings cannot be ignored. Small people cannot be ignored. They are all gifted with the potential to be realized in their own ways, serving the good news of God.
*My book on the parables of Jesus: Jesus's Truth: Life in Parables (2018).