Why does my 15 month old prefer to sleep on the floor?

when i take my 15 month old little boy to bed, i tuck him in and say night night, when i close the door, he climbers out of his bed, puts his quilt on the floor and falls to sleep on the floor! hes been doing this ever since his daddy went away to the Falkland's. i keep having to go into his room at night just to make sure hes in his bed. why does he prefer to sleep on the floor than his own bed? thank you xxxx