If you read the entire sonnet, it's pretty obvious that it's all about lust
. It is even referred to as the "Lust Sonnet."
The concluding couplet, "All this the world well knows, yet none knows well / To shun the heaven that leads men to this hell" can thus be read as an acknowledgment that we all suffer from emotional problems (as if we need to be told) that cause us to misperceive the desirable heaven of love-making as our personal version of hell.
But if this is so, why is it that so many of us do not shun a love relationship but rather seek it out and do all within our ability to sustain it? In part, the answer is that this sonnet's insistence that "none knows well / To shun the heaven that leads men to this hell" exaggerates to create its effect: it overlooks those who in fact do shun the heaven that leads to their own personal or philosophical hell. But this answer begs the question. A more satisfactory response might be that lust within the context of a loving relationship can be a pleasure even in the presence of unconscious conflicts, even though these conflicts may limit the passion or constrict the frequency, creativity, and diversity of the loving. The impact of this sonnet on so many readers suggests that this may well be a common--if not universal--situation. If this is true, then the entire sonnet may indeed reflect a wish to define: it defines sexual love in terms of both its pleasures and its problems--conscious and unconscious.
http://www.clas.ufl.edu/ipsa/journal/ar ... rims02.htm
original spelling, from the first edition, 1609
http://unix.cc.wmich.edu/~cooneys/poems ... oldsp.html