Will Byers is taken to the Upside Down because a Demogorgon targets him while he is alone on his way home, treating him as prey/food rather than for any clearly explained personal reason in Season 1.

What happens the night Will is taken

Will is biking home alone at night after playing Dungeons & Dragons at Mike’s house when a Demogorgon crosses his path near Mirkwood Road. He crashes his bike, runs home, and hides in the shed, where the Demogorgon corners him and pulls him through a rift into the Upside Down.

Why the Demogorgon chooses Will

The show initially presents Will as one of several random human victims the Demogorgon hunts to feed on and grow stronger once the portal from Hawkins Lab is opened. Some fan theories suggest Will may have been bleeding after his bike crash, which could have attracted the Demogorgon like a predator sensing blood, but this has not been definitively confirmed on-screen.

Later hints about a deeper reason

Later seasons reveal Vecna was secretly controlling events in the Upside Down, implying Will might have been especially important to Vecna’s larger plan, not just a random target. However, within Season 1 itself, the clear in-story answer is that Will is alone, vulnerable, and encountered by a hunting Demogorgon that drags him into the Upside Down as prey.