Shakespeare didn't give his sonnets titles, but that doesn't mean that the numbers we use to refer to them are random. In fact, the 154 sonnets that Shakespeare wrote are grouped together by theme, so it matters that the poem we call Sonnet 2 is the second one in the volume. It's surrounded by other sonnets about the importance of having babies (the "procreation sonnets"). Sometimes Shakespeare's sonnets are referred to by their first line. That works better for the really famous first lines, like "Shall I compare thee to a summer's day?" (Sonnet 18). We're willing to bet that only real Shakespeare nuts would know what poem you were talking about if you mentioned "When forty winters shall besiege thy brow."