Why does a sonata-form exposition in a minor key often move to the relative major key?

In sonata form, the exposition is characterized by a dynamic contrast of keys, which serves as the foundation for the movement's dramatic structure. Composers aim to move from the "home" or tonic key to a closely related but distinct key area to create an opposition that the rest of the movement—the development and recapitulation—will eventually strive to reconcile.123

When a work is in a major key, the dominant key is typically chosen for this contrast. However, when the home key is minor, the relative major is almost invariably selected. This choice is based on the principle of selecting one of the two keys most closely related to the tonic. By moving to the relative major, the composer establishes a clear tonal contrast that remains functionally linked to the home key, providing the necessary tension and structural balance required for the sonata-form process.12