For this one we're going straight to the author, who said:
In my [junior high] class were two groups of Asian boys, two cliques. The one I was a part of, [we] were either born in the states or came when we were really young, and we felt more comfortable speaking in English than in our native tongue. And then this other group came when they were much older, and they usually spoke in Chinese or Korean. I remember very distinctly feeling like we wanted people to know that we were two distinct groups.(Source)
Sound familiar? In American Born Chinese, we see both groups represented by Jin and Wei-Chen. For his part, Wei-Chen represents the full-on "F.O.B." Asian, new to America with his high-water sweatpants, "Robot Happy" t-shirt, glasses, and heavily accented, broken English (2.62). Wei-Chen is clearly foreign and different from all the American boys who run around in non-descript tees and jeans.
Then there's the Asian American boy, born and raised in America, whom Jin represents. He's the boy who speaks with your typical American accent, the one who knows all the pop culture phrases like "word to your mother" and "don't have a cow, man" (5.72) and who dresses like the other American guys—a plain t-shirt and jeans (2.29).
How else are the two different? Jin's the American boy who's fixated on the difference between himself and Wei-Chen (like our dear author was with the other group of Asian boys at his school), while Wei-Chen is all about how similar they are. After Wei-Chen tells Jin "'We're alike. We're brothers […] We're blood'" (8.118-8.119), Jin (nastily) says, "'You've got to be kidding. You and I are not alike. We're nothing alike'" (8.120-8.121).
This isn't the first time that Jin tries to distance himself from Wei-Chen. For example, on Wei-Chen's first day at school, Jin refuses to speak in Chinese with him, telling Wei-Chen "'You're in America. Speak English'" (2.62). Moreover, he refuses to be Wei-Chen's friend and says, "'I have enough friends'" (2.65-2.66).
With all of this insistence on difference in mind, Jin learning to become a true friend of Wei-Chen is about something bigger than just a typical friendship between boys. It's also a fantasy of a united Chinese brotherhood: two types of Chinese boys, who—despite their different cultural and national backgrounds—can combat the overwhelming desire to be "white"/mainstream Americans and revel instead in their Chineseness.
That's why the aww moments of Jin's and Wei-Chen's friendship typically occur over Chinese/Taiwanese stuff: Wei-Chen's toy robot/monkey (2.71-2.75) and pearl milk tea at a Chinese bakery/café (9.125).