Solar astronomers have been trying to answer this (very good) question for a long time and still don't know exactly what causes flares and the 11 year solar cycle. Flares are caused by the explosive release of magnetic energy on the sun. There are more flares when there is more of this magnetic energy around. Every 11 years, the magnetic energy increases and this causes the increase in the flares. So that is part of the answer but we still don't know why this magnetic energy increases every 11 years. Besides flares, magnetism on the sun causes sunspots and plages. So when there are a lot of sunspots on the sun you can expect more flares. Astronomers actually count the sunspots every day and make graphs of this sunspot number. It shows the 11 cycle very clearly. In fact, astronomers could count sunspots before they even realized that flares happen. So we have these sunspot numbers now for over 200 years.
Flares, especially big ones, can affect us in several ways. They can cause some static on your radio or TV, create aurorae (Northern lights), and sometimes cause power outages. For astronauts in space, a flare can even be dangerous but on earth we are protected by the thick layer of air above us.
The magnetic fields, which cause all this, are generated deep in the sun and actually have a 22 cycle because the polarity (positive/negative) of the magnetism reverses every other 11 year cycle. Most solar astronomers think that the field is generated by what is called a dynamo effect caused by the sun's rotation. The earth's magnetic field is also caused by a dynamo but it is much more steady than the sun's.
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