Autistic children typically fail to some degree to recognize other people as being a human entity like they are. In extreme cases other people appear to be no more significant than say a chair or a television. This means that for autistic children, the only significant person in the world is themselves and as a result they behave in a manner that appears to be very selfish.
Where non-autistic children know instinctively that to get along with other children they must consider the opinions and feelings of others, autistic children must be taught this concept. The idea of being considerate often begins with something as simple as taking turns.