This may be a question better suited for /r/askanthropology

In some respects, the Aztecs both venerated and feared the sun. But first, a disclaimer: as with many things relating to Aztec mythology, this is disputed, vague, and varies fairly often.

Unlike many cultures, the Aztecs considered the sun to be finite. As an extension of this, the sun itself is the fifth such sun to exist. And, according to the Aztec creation myth, the moon was also a sun, right up until the other gods threw a rabbit (yes, a rabbit) in front of it. But, perhaps because there were supposedly two suns rather than one, the sun was weak and dim.

So the gods gave their blood to the sun. You can probably see where this is going.

Basically, the blood offering the gods gave to the sun wasn't enough to keep the sun strong. And since the gods symbolically offered their blood to the sun, the Aztecs themselves started symbolically offering the blood of humans to the sun (only, y'know, there was nothing symbolic about having your heart cut out). The Aztecs presumably believed that this human sacrifice would, in some respect, mirror the blood offering the gods made to the sun in the past and, therefore, the sacrifices would keep the sun strong.

It should probably be noted that in all four of the previous epochs as recognized by the Aztecs, the death of the sun occurred alongside the death of the planet. The Aztecs, therefore, believed that failing to cut enough hearts out would result in the sun dying which, in turn, would result in everyone dying. Human sacrifice was supposedly the lesser of these two evils. The exact nature of the sun in Aztec mythology varies (since "Aztec" sometimes corresponds to the broader Nahuatl area rather than just the Aztecs themselves), but the recurring theme is that if you don't offer up enough human sacrifices, the sun will die/stop moving/go out/explode/whatever and everyone will die.

You can read more here, here, and here.

Edit: sun, not son. l2homophone, past me.

I asked a similar question a couple of weeks ago however it was removed because it was a "throughout history" question and was told it needed to be more specific. What gives?

I live on the island of Maui. We grow up hearing the Hawaiian folk tale about Haleakala (House of the Sun) -- our dormant volcano -- which is where the demi-god Maui throws a net to capture the Sun. He wants the sun to slow down because it travels too fast for his mother to dry her kapa cloth. The sun is basically a jerk and in a hurry, so doesn't want to slow down. Maui has to catch it and he doesn't release the sun until it agrees to slow, then it goes slower in the summer and faster in the winter. Here are some some sources. 1, 2