The velocity of the body decreases when the body is projected upwards because

When the body moves up, its speed will decrease uniformly, becoming zero at the highest point. As the body moves down the speed will increase uniformly. It will return with the same velocity with which it was thrown up. 

The velocity of an upward-moving body decreases because gravitational acceleration works on it downwards. For a body projected upwards, the velocity-time graph is of the type shown in the figure.