The sonnet uses Iambic Pentameter. Poems that use iambic pentameter have 10 syllables for each line, and there's a stress on each second syllable (like a heartbeat).
Iambic pentameter is usually used to add rhythm to poems and mimics a human heartbeat, which represents words being spoken from the heart. Shakespeare uses iambic pentameter when his characters are speaking with emotion.
In this case, iambic pentameter is being used to emphasize the emotion of the speaker's words and add emphasis to the poem in general.