It's possible (and more likely than you might imagine) that the growing
conditions on your lawn are more friendly to weeds than grass. This means
that you may end up poisoning your land every year, which is bad because the
chemicals you need to use have not been, and cannot be proven safe.

Have you spoken with your state's cooperative extension about soil tests,
and ways of helping the grass compete with the weeds? And, what are your
expectations? Perfection, like a golf course?