Some musings on the sonnets of Shakespeare

I was translating some sonnets of Shakespeare into Tamil. I was wondering why the bard thought it so important to advice a young man like me to marry without fail and to impress on the mind of the lad the sanctity of wedding and begetting. Was it perhaps to himself? Was it a soliloquy? Naturally talking about marriage engenders considerations of immortality and furtherance of life which otherwise ends with the individual. People have speculated long about the dark lady, the young man, even a Lord and what not. But why a poet like Shakespeare should dilate so much on a predominantly prevalent theme like marriage for a total length of say 156 or 154 sonnets? I am trying to read in between lines and attempting various interpretations but the persistant mystery seems to be evading.
'Unthrifty loveliness, why dost thou spendUpon thyself thy beauty's legacy?Nature's bequest gives nothing but doth lend,And being frank she lends to those are free.Then, beauteous niggard, why dost thou abuseThe bounteous largess given thee to give?Profitless usurer, why dost thou useSo great a sum of sums, yet canst not live?For having traffic with thyself alone,Thou of thyself thy sweet self dost deceive.Then how, when nature calls thee to be gone,What acceptable audit canst thou leave?Thy unused beauty must be tomb'd with thee,Which, used, lives th' executor to be'
Was it death or progeny or pragmatic sensibilty which prompted W.S. or none of these but something else? I am having my mind open to any chance hints or insights from the poem or anywherelse.