Why do composite volcanoes generally have violent eruptions than shield volcanoes?

Shield volcanoes are very gentle. Their magma is runny in consistency and tends to trickle out from the top rather than erupt or explode. But composite volcanoes have andesitic magma which is high in silica content and therefore thick and gooey. Great lumps of the sticky magma cool at the top of the volcano to form a sealed cap. Meantime the magma beneath the cap builds up pressure which eventually becomes too much to be contained and all the magma violently erupts out from the top of the volcano shooting hundreds of miles into the air and in all directions. The gas and ash from the eruption come cascading down the sides of the volcano in a fatal cloud called a Pyroclastic flow.