For the first half of "Eldorado," the knight is totally alone. It is only in line 15 that he meets somebody else, and only a "pilgrim shadow" at that. The shadow doesn't seem very real, and it is possible that he's just talking to himself anyway. For all intents and purposes, the knight is alone throughout the poem, and it is implied that he will die alone. His obsession with finding Eldorado leaves him isolated, with only his dreams of Eldorado to keep him company. Super-sad, gang.
Isolation is a form of death. The knight is alone and "alive," but all the shadows in the poem suggest that death follows him everywhere. He's in major need of a buddy.
Careful, Shmoopers! If we get too obsessed with one thing, we end up isolating ourselves and—possibly—dying alone, just like the knight.