The answer that makes the most sense is "fit the meter"
Shakespeare's sonnets are written predominantly in a meter called iambic pentameter, a rhyme scheme in which each sonnet line consists of ten syllables. The syllables are divided into five pairs called iambs or iambic feet. An iamb is a metrical unit made up of one unstressed syllable followed by one stressed syllable, so it makes sense that he would want his sonnets to fit and sound good read aloud.
