The poet's teacher in the poem "Geography Lesson" dreams of sailing around the world. He has harbored this dream all his life.

Reasons for the dream:
1. Passion for exploration: The teacher is fascinated by the vastness and diversity of the world. He yearns to explore different cultures, landscapes, and experiences. His dream of sailing around the world reflects his desire to quench his thirst for adventure and knowledge.
2. Curiosity and love for geography: The teacher's dream is rooted in his deep interest in geography. He wants to witness firsthand the places he has taught about, understand their physical features, and experience the unique characteristics of each location. Through his dream, he seeks to connect his theoretical knowledge with practical experience.
3. Escape from monotony: The teacher's dream also signifies his longing to break free from the routine and monotony of everyday life. He seeks to escape the confines of his classroom and explore the world beyond, embracing new challenges and opportunities.
4. Desire for personal growth: By sailing around the world, the teacher hopes to broaden his horizons, expand his understanding of the world, and gain new perspectives. He believes that such an experience would enrich his personal and intellectual growth, enabling him to become a better teacher and mentor to his students.

In summary, the teacher in the poem dreams of sailing around the world due to his passion for exploration, curiosity about geography, longing to escape monotony, and desire for personal growth. His dream reflects his deep connection with the subject he teaches and his yearning to experience the world beyond the confines of his classroom.