The first twelve lines elaborate a comparison between music and the youth, who, should he marry and have a child, would then be the very embodiment of harmony. But music, "the true concord of well-tuned sounds," scolds him because he remains single — a single note, not a chord. By refusing to marry, the youth destroys the harmony that he should make as part of an ensemble, a family. Just as the strings of a lute when struck simultaneously produce one sound, which is actually made up of many sounds, so the family is a unit comprised of single members who function best — and most naturally — 

when working in tandem with one another.

The theme of the youth's failure to marry and to have children is continued. A lesson is drawn from his apparent sadness in listening to music. Music itself is concord and harmony, similar to that which reigns in the happy household of father, child and mother, as if they were separate strings in music which reverberate mutually. The young man is made sad by this harmony because he does not submit to it. In effect it admonishes him, telling him that, in dedicating himself to a single life he makes himself worthless, a nonentity, a nothingness.

Sweet (things) and joy are inherently harmonious, they do not fight against themselves. The construction of these first two lines is consciously melodic. Musis music, sweets sweets, joy joy. A slight air of disharmony sets in with lines 3 and 4, with 'receiv'st not glady, receiv'st thine annoy'.

Rhyming Scheme

Sonnet 8 is written predominantly in a meter called iambic pentameter, in which each sonnet line consists of ten syllables with the rhyming scheme of a,b,a,b,c,d,c,d,e,f,e,f,g,g.

Music to hear, why hear'st thou music sadly? (a)
Sweets with sweets war not, joy delights in joy: (b)
Why lov'st thou that which thou receiv'st not gladly, (a)

Or else receiv'st with pleasure thine annoy? (b)
If the true concord of well-tuned sounds, (c)
By unions married, do offend thine ear, (d)
They do but sweetly chide thee, who confounds (c)
In singleness the parts that thou shouldst bear.  (d)
Mark how one string, sweet husband to another, (e)
Strikes each in each by mutual ordering; (f)
Resembling sire and child and happy mother, (e)
Who, all in one, one pleasing note do sing: (f)
Whose speechless song being many, seeming one, (g)
Sings this to thee: 'Thou single wilt prove none.' (g)

In my opinion, this sonnet is written wittily as it has succeeded in conveying the symbol of music into the concept of having a family. However, I am only partially agree on the idea on the idea that you will only find true happiness when you have a family on your own. It is true that by having to share your life with your significant other and having a family together will make you a happy man but it does not necessarily guarantee your happiness in life. Perhaps the burden of marriage's responsibilities would also make you feel at the edge of your sanity.