What signals the mood change in a sonnet?

In a Sonnet, mood changes are often signaled by shifts in tone, imagery, or diction. These shifts can occur at the volta, or the turn, typically found at the start of the third quatrain in a Shakespearean sonnet or after the octave in a Petrarchan sonnet. The mood change can be used to introduce a new perspective, idea, or emotion that contrasts with or builds upon the preceding content.