The crossdressing urge definitely comes and goes for me. It seems to depend on how engaged I feel with my "normal" life. It never goes away completely, but if I'm on vacation, say, there's less of an urge.
In talking with my therapist, I've come to the conclusion that it's kind of a "place" for me to escape when life feels too humdrum. When I dress up all the way -- wig, makeup, etc. -- I feel a high that is more powerful than anything else I have ever experienced. If I'm able to do it over the course of a few days, then the intoxication scales back to a happy feeling of well-being even though it's become routine. And I have never been glad when the end of a period of dressing up arrives.