It is usually for the first line of a sonnet not to have a comma or a full stop at the end of it. This first line runs on to the next line . Why do you think Shakespeare did this ?
The opening lines of the sonnet dive the reader into the theme at a rapid pace, accomplished in part by the use of enjambment - the continuation of a syntactic unit from one line of poetry to the next without any form of pause, e.g., "Let me not to the marriage of true minds / Admit impediments ..." This first quatrain asserts that true love is immortal and unchanging: it neither changes on its own nor allows itself to be changed, even when it encounters changes in the loved one.
