It can mean a lot of things. Sometimes women get exhausted in their daily chores and don't feel up to intimacy. Sometimes they carry bad feelings about something that happened weeks ago. Sometimes they just need a little excitement and romance. Try a mini-vacation. Go away from home, someplace that she would like, and try to understand what it is that SHE needs. If all else fails, you may need to go to a marriage counselor to work it out.