Why did the aztecs think it was necessary to make blood sacrifices to the sun god, huitzilopochtli?

The Aztecs thought it was necessary to make blood sacrifices to the sun god, Huitzilopochtli because they believed that they needed to appease the sun god via human sacrifice. The Aztecs did not only offer human sacrifices to the gods. They also sacrificed animals and they cut themselves as a blood sacrifice.  If they did not offer human sacrifices or any blood sacrifice, the sun god would get angry and punish them. The punishments would include failed crops and natural disasters.