No one really knows what actually causes a migraine. There are a few theories, and there certainly appears to be some merit in the belief that swelling or constriction of vessels and capillaries in the brain takes place. 

Blood vessels either narrow or expand. Narrowing can constrict blood flow, causing problems with sight or dizziness. 

When the blood vessels expand, they press on nerves nearby, which transmutes into severe pain via the central nervous system.

Another theory focuses on chemical changes in the brain. When chemicals in the brain that send messages from one cell to another, including the messages to blood vessels get narrow or expand, are interrupted, migraines can occur. Hypoglycaemia or low blood sugar may also cause migraines. 

However, experts now tend to believe that migraine starts with an underlying central nervous system disorder, which, when triggered by various stimuli, sets off a chain of neurologic and biochemical events, some of which subsequently affect the brain's vascular system. No experimental model fully explains the migraine process.

There certainly seems to be more evidence of what triggers a migraine, than what the actual cause is. 

Triggers - Physical Factors...

What might trigger a migraine in one individual may have little or no triggering effect in another. 

Activities, such as prolonged exercising or sport, or overexerting by straining, might be the beginning of trouble for one sufferer but any or all of those may not bother another migraine sufferer at all. 

Migraine headaches can be triggered by a number of factors. Diet, tension, medications, eyestrain, food or chemical allergies and fatigue are among the most common with causes varying widely among individuals. Some foods commonly seen as migraine triggers are chocolate, red wine, cheese and foods using nitrites as a preservative. 

Environmental Factors...

Some people are sensitive to weather conditions, and as such, these can cause biological changes in the body's chemical balance and thus precipitate a migraine headache. Even just a change of daily routine can cause the onset of a migraine attack. 

Weather conditions also can increase the severity of a migraine triggered by other factors. Dry and dusty atmospheric conditions may also cause the onset of a migraine.

Overcrowding in a badly ventilated room, train or bus might trigger an attack in some migraine sufferers. Other environmental triggers include bright sunshine or glare conditions which sufferers can find unbearable after a short period of exposure.

Pollution.....

It is highly probable that pollution in the atmosphere and waterways, which in turn, ends up in our drinking water supply and food supply has been largely responsible for the massive increase in the number of reported cases of migraine headaches, the world over. 

We cannot continue to keep spewing deadly toxins and pollutants into the environment, without there being huge consequences to the state of our health. 

Smoking...

Self-induced factors, which can trigger migraine attacks, must include smoking tobacco or other substances. The "up close and personal" affront to one's body by such highly toxic pollutants, has to be considered not only, extremely dangerous, but totally detrimental to the life expectancy of body and brain cells and how they function, the consequences of which, can only be terminal. 

It is important to understand that the chemical imbalance and constriction of blood flow to parts of the brain is most likely the very reason you suffer a migraine. Therefore, to induce clotting, and to destroy blood vessels and cells, can only bring about more pain and suffering, which has been proven to result from smoking.

Drinking...

Excessive drinking of alcoholic beverages can cause the onset of migraines. A hangover can easily continue on for more than twenty-four hours as a migraine. 

Therefore it is imperative, in order to prevent this type of triggering effect, at the very least, be self disciplined enough to restrict oneself from over-indulgence. 

Some people have to refrain completely from certain types of drink, as those types are a trigger in themselves. One of those is red wine. Many beers have excessive amounts of chemicals in them in the guise of ingredients, more so in the case of "light beer". Trying a naturally brewed beer, in moderation, may make a difference! 

Stress...

Stress is a relatively recent phenomenon, in that, it really only began to be introduced into the general vernacular from the very late 1970s onward. 

Of course people generally, in this day and age, are pressured into a lifestyle that, despite us supposedly being more intelligent than any generation before us, are duped by slick propaganda all around us, into believing that we "must have" the latest and greatest of every consumable product on the market. 

It is often the case that both parents in the family work long hours, skip meals or eat on the run, with no time to properly look after their health. We spend less quality time with our children and so on and so forth, just in order to fund a lifestyle that meets "society expectations". All this, of course, equals elevated stress levels in most of us.

Perhaps more importance needs to be placed on this very problem, and living a life without stress should be high on every migraine sufferer's agenda. Also emphasis needs to be placed on preventive maintenance.

The catch 22 in the whole deal of course is that the worse your migraine is, the more anxious you can become about impending attacks, therefore, creating stress. It can become a vicious cycle.