Why does the velocity of a body thrown upwards decrease until it reaches the maximum height?

When you throw a body upwards from the earth's surface, its velocity starts decreasing until it reaches the maximum height (at which point its velocity is zero). Then, it begins falling downwards and the velocity starts increasing. This happens due to the gravity of the earth. In other words, the earth is exerting an attractive force on the ball and is pulling it towards itself. This is the reason the ball falls down.

We can also describe the solution using the equation of motion:

v = u + at

and v^2 = u^2 + 2as 

(v is final velocity, u is initial velocity, a is the acceleration, s is the distance traveled and t is the time taken).

In the case of a ball thrown upwards, the value of 'a' is equal to the acceleration due to gravity (g) and has a negative sign (-9.8 m/s^2). That is, for a ball thrown upwards,

v = u - gt

and v^2 = u^2 - 2gs

Hence, the earth's gravity opposes the upward motion of the ball, causing the velocity to decrease until it reaches zero.

Hope this helps.