Music to hear, why hear'st thou music sadly?
Sweets with sweets war not, joy delights in joy:
Why lov'st thou that which thou receiv'st not gladly,
Or else receiv'st with pleasure thine annoy?
If the true concord of well-tunèd sounds,
By unions married, do offend thine ear,
They do but sweetly chide thee, who confounds
In singleness the parts that thou shouldst bear.
Mark how one string, sweet husband to another,
Strikes each in each by mutual ordèring;
Resembling sire and child and happy mother,
Who, all in one, one pleasing note do sing:
Whose speechless song being many, seeming one,
Sings this to thee: 'Thou single wilt prove none.'
Sonnet 8 is a procreation sonnet by William Shakespeare, urging the young man to whom it is addressed to marry and have children. A comparison is made between the harmony of different instruments in an orchestra, voices in unison (although on "one note" an octave apart) and a harmonious relationship between a family.
The music, which he hears, angers him as it makes him feel worthless living a single life. The last line "Sings this to thee: 'Thou single wilt prove none" implies that he will become nothing having not had children. Singleness cannot make a unison (see sonnet 128 and Fred Blick, reference below). As Blick points out, this sonnet, which in its numbering invokes the union or unison of the octave, is associated with sonnet 128 by the vocative naming of the addressee as "music", but in sonnet 128 harmony in unions/unison is to be achieved by the "kiss", not marriage.