The Asuras did not like Indra at all. It's not because he's not as talented as his brother, or because his brother is more favored by his father than he is.
But...
Indra didn't even read the love letter written by Asura.
Asura is angry!
Asura's anger is maxed out!
Asura said that he should ignore Indra!
So Asura fell into the river and came to the human world.
Indra: Where is my brother... QAQ