misunderstanding psychology
Rei inadvertently stomped on Tyson's abandonment issues, and Tyson felt like revenge. It was a main characteristic of their relationship: neither of them wanted to be left by the other first, so they'd slowly part emotionally and then suddenly part physically, scared, self-defensive, perpetuating their cycle of dangerously unstable friendship.
Tyson liked to forget that Rei existed, and wouldn't notice him, anywhere—for weeks. He very effectively undermined their emotional connection, their friendship, and the silence made Rei paranoid. All 'cause Rei'd gotten a letter from the 'Hills again.
Tyson, keyed up, kept note that Rei was around, however, checkin' his mail, backin' him up, because upon noticing that Rei WASN'T around… backing him up… Tyson flipped fricking out. Raved about cryptic, unreadable Chinese letters. Hilary cited abandonment issues and everyone glared at Hiro.
Rei heard about Tyson's hysteria by mail, and looked suspiciously around at the 'Tigers and decided they didn't love him quite as much as Tyson had to, to be freaking out so much… so he came back home and was Tyson's best friend for a couple of glorious days.
Until Rei got mail and Tyson started forgetting.
And it NEVER stopped. And they were friends forever.