Why does Amir not enjoy the lavish birthday party thrown in his honor?

The birthday party that Baba has for Amir, which occurs in Chapter 8, is for his 13th birthday.  This seems to be a kind of "coming of age" party, since it is so lavish and large.  There are at least three reasons that Amir takes no pleasure in this party, one being that it is not really for him, another being that Assef comes to the party, and another being that Amir's pleasure in anything...

The birthday party that Baba has for Amir, which occurs in Chapter 8, is for his 13th birthday.  This seems to be a kind of "coming of age" party, since it is so lavish and large.  There are at least three reasons that Amir takes no pleasure in this party, one being that it is not really for him, another being that Assef comes to the party, and another being that Amir's pleasure in anything is tainted by his guilt. 

Amir says, as he scans over the responses to the invitations, that he does not even know three-fourths of the people who are coming to the party. He realizes that they were not coming because of him, but because of Baba. He says, "It was my birthday, but I knew who the real star of the show was" (94). His father is an important man in the community, one of the ruling class, really, and this party is a means of demonstrating that.

When Assef appears at the party, it is truly ruined for Amir.  Assef is the bully who has raped Hassan, the bully that Amir was unable to face down because of his own cowardice.  Assef is also a neo-Nazi who torments Amir with his gift, a biography of Hitler.  Also contributing to Amir's unease at Assef's presence is the fact that Baba seems to find Assef to be an admirable young man, one he would like Amir to associate with.

Finally, Assef is a reminder of Amir's guilt in not trying to save Hassan, and this guilt taints any enjoyment that Amir might have otherwise had at the party. As the chapter ends, he sees Hassan serving some beverages to Assef and Wali, another bully who was present at Hassan's violation, and then "Assef grinning, kneading Hassan in the chest with a knuckle" (160), once again tormenting Hassan. 

All of these factors contribute to Amir's inability to enjoy this large and lavish party. It is not really for him at all, Assef is enough to ruin any party, and Amir is plagued by his guilt.