Brilliant question. Actually, it's very common for asthma to appear to disappear for days, weeks, months and even years. In the past it was believed that asthma was a childhood disease that went away when a person grew up. We now know this is not true.
While you may be having no asthma symptoms, your asthma is still there, hybernating if you will. Asthma can appear to disappear for days, weeks, months even years. Why this happens is a mystery.
There are some theories for this. One is that you are no longer exposed to your asthma triggers. Or, perhaps your lungs are larger and better able to handle your asthma triggers.
Chances are you still have some very mild degree of inflammation in your air passages, only this inflammation is not very sensitive to your asthma triggers anymore, and making it appear as though you no longer have asthma.
Be thankful that you are no longer having asthma symptoms and enjoy your life. Yet at the same time, don't forget what it was like to have asthma and always be aware that you do still have it. What I mean by this is stay asthma wise, continue to see your doctor at least once a year, and by gosh don't ever start smoking.