He tries to stay away from Hassan because he does not know how to deal with the feelings of Hassan's rape
Amir underestimates Hassan's loyalty and selflessness, believing that their friendship is one-sided and that Hassan is lesser than him due to his low socio-economic status as a Hazara. Amir also underestimates Hassan's courage and strength, not realizing the depths of his character until a pivotal moment in their lives.
Amir is selfish and Hassan is humble and loyal to Amir. Hassan is a Hazara, Amir is a pashtun. Amir is granted to right to get an education, Hassan is not.
Actually, they are half brothers. Amir had found out that when Rahim Khan had hinted that Ali was not Hassan's father but Baba was (which explains why Baba had a closer relationship to Hassan and Amir).
Amir frames Hassan for stealing money and a watch, knowing that Hassan will take the blame to protect him. Amir stands by and does nothing to stop it, thus indirectly getting both Ali and Hassan in trouble.
Amir threw pomegranates at Hassan out of frustration and jealousy stemming from their complex relationship and his own insecurities about his father's affection for Hassan. This act was an expression of Amir's conflicting emotions and desire to hurt Hassan in the heat of the moment.
Amir and Hassan, the sultans of Kabul
Hassan and Ali give Amir a leather-bound notebook for his birthday.
Hassan's first word is Amir. Amir does not reveal his name till this point, which shows his underlying guilt because Hassan loves him so much and Amir's betrayal.
Amir put his watch and some money in Hassan's mattress. Hassan found these items and told Ali about how he got raped while Amir hid and watched, so they left Kabul.
Baba shocks Amir by letting Hassan and Ali stay at the house, as Amir had put the wristwatch and Afghani bills under Hassan's mattress so that when Baba did find out about Hassan's stealing (a trait that Baba once told Amir he would never forgive in a person), Baba would want to push them out. Instead of Baba kicking them out, he forgave Hassan for "stealing" (he only admits stealing from Amir to protect him), and begs them to stay. Hassan and Ali leave the next morning and Baba weeps when they leave.
Amir feels jealousy towards Hassan when Baba shows favoritism towards Hassan, when Hassan excels at kite running, and when Hassan receives attention and affection from Baba. These instances highlight Amir's deep-seated feelings of inadequacy and envy towards Hassan despite their close relationship.
Hassan is Amir's half brother. Hassan's father, Ali, was like a brother to Baba so he lived with Baba. However, Baba impreganated Ali's wife and had Hassan who became good friends with Amir.
Amir thinks it is ironic because Hassan smiles everyday even with the inability to do it correctly. Amir also see it is ironic becuase Baba (Amir's father) pays for the surgery when Hassan is seen not to be Baba's son. (even though Hassan is Baba's illegitimate child)