Why are ideas of love, friendship, and marriage the subject of many Shakespearean sonnets, such as 30, 55, and 116?

Scholars believe that Shakespeare's sonnets were based on his personal relationships, namely with the Fair Youth (Sonnets 1-126), and, therefore, deal the subjects of love, friendship, and marriage.

Sonnet 30 addresses the Fair Youth, a young nobleman who Shakespeare was friends with and some believe was romantically involved with. The poem begins with a dejected tone, recalling when the speaker, presumably Shakespeare, remembers and cries for his deceased or lost friends. However, in the couplet he states that when he thinks of his "dear friend, all losses are restor'd and sorrows end" (13-14). Friendship is important enough to Shakespeare that it cheers him up.

Sonnet 55 is also presumably about the Fair Youth and Shakespeare's attempt to immortalize his friend in this sonnet after the youth has perished. In the couplet he states, "live in this, and dwell in lovers' eyes" (14) meaning that people in love, such as the youth and Shakespeare (either romantic or platonic love), will be able to read about him for centuries to come.

Finally, Sonnet 116, is about true love and the fact that it doesn't change according to circumstances outside of it. He begins the sonnet with, "Let me not to let me not to the marriage of true minds admit impediments" (1-2), which means that nothing should stop the joining of two people in love. The speaker also recognizes that people will change, but that this should not change true love. In real life, Shakespeare was getting older, so his looks were fading. He also knew that the Fair Youth would not be young and beautiful forever.