It may seem easier to acquiesce than to find out what's really bothering your child.
Sleeping in one's own bed can be pretty scary for a child. When the lights go out, all that was familiar in daylight is now unrecognizable.
If your little one doesn't want to go to bed alone, coax your child into explaining what's bothering him. You can provide him with a flashlight or nightlight. A large stuffed animal might allay his fears of being alone. You can also ask your child for suggestions of what might make his own bed (or bedroom) feel more comforting.
Dr. Tanya Altmann shares tips on how you can get your child to sleep alone.