Question

Why is a chair conformation of cyclohexane more stable?

Answer 1

Because it minimizes so-called trans-annular interactions.

Explanation:

In a chair conformation, each face of the cyclohexane ring has 3 axial substituents, whose interaction is sterically unfavourable. This is to be compared with boat and twist-boat conformations whose substituents tend to eclipse each other to give unfavourable steric interactions.

This answer is no substitute for actually making a model, and observing trans-annular interactions in three dimensions.