Query: What makes a woman passionate about a man?????

Ladies, this is an honest attempt to figure out what it is that extinguishes the passion in a marriage.  Specifically, in a marriage where both people are friends, there is no abuse, no infidelity or other such problems and two wonderful kids.
 I understand that marriage is a complicated relationship, but I'm trying to hone in why my marriage has lost its spark.  My wife and I are going to counseling, and she is really trying, but she is still not feeling any passion towards me after 14 years of marriage.  I shouldn't say "still" because it hasn't been 14 years, perhaps just the last couple of years.  According to what she tells me our sex life was good, I'm a great guy, but there is just something lacking. 
Last night, she put on her sexiest outfit, lit some candles, and everything was going well, but..... when it came to the moment of truth, she "couldn't".  I am not pressuring her; she wants to get that feeling back; but something just isn't there.  This question ISN'T about sex.  I think sex is more of a symptom of a lack of passion (or if I'm wrong, please let me know).  I'm just a confused guy, trying to figure out what I can do.
So, the query again, what makes a woman passionate about a man?  I'm sure there is a lot of good insight out there, and a lot of men who could use it, so all thoughts would do a lot of good.
Thanks in advance!