So few men understand why the women they date lose interest. Chances are, the women who lose interest don't understand why, either.
Fortunately, I have a generally good idea of what causes this bizarre phenomenon. But rather than take credit for this myself, I'll simply refer you to a handy little set of dating advice columns (<a href="http://www.askmen.com/dating/doclove/index.html">clicky</a>). This guy really knows what he's talking about, I think. The main point he usually makes is that you have to make the woman chase you, not just in the first few weeks, but throughout your relationship, hopefully until one of you dies.
Read through his stuff. The answers will come.