Well, your sex life can vary greatly depending on numerous factors. Sam has a valid point, she may just not think it's that important to you. Also, the amount of time you're at work or doing other activities can affect your sex life. If you work 30 hrs per week (Each of you) you're a lot more likely to have more sex than if you both work 60 hours per week. Work, school, working out, hobbies... these can all affect the amount of sex you have. Let's face it, as fun and pleasurable as sex is, it is also strenuous. When you are tired, you are much less likely to want sex.
If you don't believe any of these are your issue, talk to her. Tell her how much you love her and how you enjoy making love to her. Let her know you would like to have more intimate time with her. Also, ask her if there is anything you havent done together but she would like to try. Don't be afraid to ask if there's anything you can do differently for her. You have to make sure you are both having as much fun as possible in order to want to have sex.
I hope this helps.