Why don't you understand?

Izaya and Shizuo are fighting as usual. But then Izaya starts to think about why does shizuo not understand why he still teases him. is it becaus he's a protozoan or an ignorant buffoon? or is it because he just literally hates him?
Izaya remembered when the went to the same elementary school. they were in different classes but he would talk to Shizuo whenever he was hurt emotionally. izaya fell in love with Shizuo, but in high he didn't remember him at all.