a lot of times I notice children that have this habit are only children, or a little spoiled...in the sense that they get what they want pretty much when ever they want...therefor the idea of having to wait for the use of a toy when someone else has it, doesn't make any sense to them, they think they have a right to take it any time they want. I have watched a lot of children that behave this way with the other children also portray the same behavior with Mom or Dad at pickup...thinking they can have what ever they want any time they want, and Mom or Dad will give them what they demand. or they are an only child, therefor the concept of having to share with others is foreign to them. Even at the age of 2, I always attempt to redirect the behavior. If they walk up to a child and take a toy from that child I will turn them around and have them hand the toy back while speaking for them..."Sorry, I took the toy, I will wait and we can share", this way not only does the taker learn through redirection that he can not take toys away from others, but by me, speaking for the taker, I also let the child the toy was taken from feel validation and understand they too are responsible to share when they are done. Redirection seems to almost always work. and modeling behavior, show the taker how to share, give a toy to another child, have the child thank you for the toy and praise them for practicing good manners. Asking the parents to work on these things at home always helps.
In regards to children learning to wait for things, and learning not to expect to "have" every time they want...I also do such things as...make them wait a minute or two for wanted things such as snack, and a glass of water....showing them they don't always get what they want when they want and ultimately instilling patience. wait for things you want, don't expect them when you command.