The bull symbolizes fecundity. That’s why people used to sacrifice this animal. Sacrificing the bull is also the overcoming of the animal instinct. It separates you from the mother. The Egyptian goddess Hathor is the mother of the gods and Taurus in the zodiac is being ruled by Venus (love, beauty and harmony). It makes the bull the feminine principle, the anima.
The bull is also the god Mithras and the god Dionysos. By sacrificing the bull, you sacrifice these gods. So it is a divine sacrifice. The living bull is the hero (Ego) and you sacrifice it to the terrible mother (the unconscious). The sacrifice is needed because the schism between the conscious/Ego and unconscious has become too large. They need to be put back in harmony and this can by done by sacrificing the Ego, because its psychic energy will flow to the unconscious. The psychic energy, or the soul of the hero, is the blood of the bull and it feeds the snake (mother) in the unconscious. The hero resurrects (Christ) and becomes immortal, like the sun. But this sacrifice can only be a self-sacrifice. The hero needs to reunite with the terrible mother on a voluntary base. He goes to the womb of mother nature and a part of him dies. The other part of the hero will be reborn on a spiritual level.
In the painting the bull depicts the Greek god Zeus who abducts Europa. This was a bit of a naughty bull.