Though the two points are reasons for someone to not share, the other reason could very well be that a person is an introvert by nature.

Another article linked by Rajesh (Marshall Goldsmith on Empowering Others) raises another possible reason: "one of the problems with smart people is their need to add a 'lot of value' to the other party." So, someone may think of sharing something but might think it is not ready yet and keep trying to increase the value of.