After having some attacks myself, I consulted a doctor (who himself is Asthmatic) and he explained that asthma is simply a reaction by the body to some outside stimulus such as cold air, allergies, viruses and/or exercise.
The severity of your attacks (or reactions) will normally depend on the amount of stimuli.
I, personally, have allergies and have had a couple of attacks in 5 years. My son, on the other hand, has attacks more frequently during the fall through spring because his are virally induced (a head cold goes right to his chest).