Vertical motion under gravity

It's all because of conservation of energy. You throw a ball upwards from ground with velocity $v$ where you have only kinetic energy

$$T=frac{1}{2}mv^2$$

after some time ball reaches it's maximum altitude (point where all of its kinetic energy transformed to potential energy) this is the point where velocity of the ball becomes zero and it now has only potential energy. $T$ and $V$ terms are kinetic and potential energies respectively

$$frac{1}{2}mv^2 = mgh$$

One can also get $v=sqrt{2gh}$

The object is accelerating in a downwards direction under gravity. That is, its velocity is increasing in the downwards direction. If the initial velocity is in the upwards direction this will reduce as the object slows down, become zero then increase in the downwards direction as the object falls faster, accelerating under gravity.

An analogy can be made with a vehicle. Imagine filled with fuel and moving forward on a road, it can move forward till all the fuel's burnt up.

Similar to that, when you throw up a ball, you are giving the ball some 'fuel' as kinetic energy. However, this time rather than it being combustion that depletes the fuel, it is the gravity that depletes it by deaccelerating the body.

This can be derived from the premise of newton's law of inertia, which states that unless an external force is acted, the state of motion of a body can not change. So, if the body were to reach a zero velocity state beginning at a non-zero velocity state, it'd suggest the presence of some external force.

The force of gravity is pulling the object down.

When the object is travelling up, assume it has positive velocity. Gravity is pulling the object down, and since it's the only force acting on the object, it will accelerate the object downwards. In other words, the object will slow down, meaning the object will simply travel slower and slower as the velocity is decreasing.

After some time, the velocity decreases to $0; rm m/s$. That's the moment when the object is instantaneously at rest. Gravity however continues to pull downwards, and so after that instant, the object will now have negative velocity, and speed up whilst falling down.

Well, in smaller terms, its like something is pushing the object in the opposite direction of its velocity, downwards until its velocity becomes 0. Then the object falls down with an increasing speed.