Because he doesn't get together with anyone in the end. Although the author hints towards it, it never happens.

Because the guy's lover gets killed in the end! I felt like crying.

Later we found out Nathan was constantly sexually molested and raped by his drunk father. And when Nathan and Roy were intimate, Nathan made moves that made Roy suspicious. Roy questions Nathan where Nathan learned to do all this and became angry.

During one night Nathan ties a string at the door of his bed room and in the middle of his sleep, his dad trips on the string. Nathan then rushes out the door and decided to spent the reminder of this nights in the grave yard where Roy and him hanged out.

One night Roy finds Nathan and offers Nathan a place to sleep in Roy's barn house. Thus their relationship mends and Roy no longer question Nathan about the details. Roy decides to take Nathan on a camping trip to a haunted house with Roy's other two friend. One of Roy's friend is Randy, a chubby kid. The other friend is Burke, a strong muscular and hairy guy who once beat up an ex marine.

While visiting the abandoned haunted house, Randy and Burke catches Nathan getting oral from Roy. Roy leaves embarrassed and leaves Nathan alone. Later Burke knocks out Nathan, carries him to the top floor where he beats Nathan and force Nathan to give him oral Later Burke rapes Nathan, and Nathan remembers the same way his dad treated him through out the years. After Burke finishes, Burke strikes Nathan twice on the head with a chair handle.

Nathan feels his heat crack open and starts to bleed and faint. Now, from Nathan's perspective Roy stands over his body, crying. The rest is a little blurry as whether it was Nathan's imagination or Roy's imagination or divine intervention or actual event, but Nathan comes back to life after the funeral ceremony and Roy sees Nathan in the forest. Roy runs towards Nathan and they escape into the forest. They rests near each other until the leaves bury them in half. And Roy says they should head North and never come back.

So I think Nathan didn't die, he was just wounded really badly. He later recovers a bit and had enough energy to walk out of the crime scene and see Roy. In the book, while embracing in the last scene, Roy sees fresh blood on Nathan's head, meaning Nathan is slowly dying. So at the end, Nathan says he will rest on the floor for a while and they embrace each other and watch the leaves fall. Obvious to me that Nathan died now, really. And Roy who knows.