Why battery short when we touch battery positive terminal to negative terminal directly ?

Before coming to find out about the short circuiting of a battery let us first try to understand about the basics about a battery. A battery has two electrodes in between which there is an electrolytic material. Due to the chemical reactions some electrons get on one electrode and there is deficiency of electrons on the other electrode. The one with electrons on it is the negative terminal of the battery while other is the positive terminal. Please do not get confused with the protons as they have no role here. Protons are the elementary particles residing in the nucleus of the atoms.
When we connect a load say a bulb or some electrical resistance then electrons will try to move through this load and an electrical current will flow which may glow the bulb also. As per the scientific convention the direction of current is taken opposite to the flow of electrons.
A battery is a source of electrical power and we can use it for various purposes by a controlled flow of the current in the external circuit. If we simply connect the two terminals with a wire then as the resistance of a wire is low, a high current will flow and not only the wire will burn out but battery may also get damaged as undesired high current flows through it. For example if we short the phase and neutral points of our electric supply in our houses then the mains fuse will blow out because of that high current.

When the positive and negative terminals of a battery are connected using a wire which is having a very low resistance a high current will flow, resulting in the release of a large amount of energy in a short span of time. This can damage the wire and also the battery. Sometimes very heavy flashes also come and it will be like fireworks on Deepavali day.
Short circuits can result in the release of high heat and it will increase the h temperatures. By any chance, if a high-voltage capacitor is short-circuited by a thin wire, the wire may actually explode due to the dissipation of high energy,
A battery is a device that will be used to produce some electrical energy. There will be two electrodes in a battery which are called anodes and cathodes. These two will be immersed in electrolytes the electrons from the cathode will travel towards the anode. Here there is no role of protons. Protons are stationary in the nucleus of the atom whereas electrons are moving. All chemical reactions will take place with the transfer of these electrons only. The elections will flow in the opposite direction to that of the current.

The Ohm's law which shows the relationship between Current, Voltage and Resistance is :
E= C R i.e the voltage E is the product of current C and resistance R.

In the question asked above, when you connect the positive and negative terminals directly(without any load in between), the resistance is NIL or very very low. So the voltage E is divided by zero or a very small value. Thus the result will be a high figure.
That means, the current will be very high, which the conducting wire may not be able to tolerate. So there occurs sparks and even the conducting wire may get damaged or even the battery damaged.

In normal circuits, in earlier days safety fuses were provided whose fuse wire would melt when high current (beyond permitted level)flows and thus breaks the circuits and saves more damage.(Nowadays MCB s are largely used).

In order to achieve the flow of current, electrons must flow from anode to cathode where these electrodes are immersed in a suitable electrolyte.
In case, a circuit is formed connecting both the terminals ( positive and negative) directly, the resistance of the connecting wire would be negligible and by applying Ohms Law, which states that the current within the system is directly proportional to voltage difference within the circuit and the flow of current is inversely proportional to the resistance of the wire.
Mathematically we can say that C = E1 - E2/R where E1 is equal to the positive voltage and E2 being the negative voltage. In the abbreviated form, it is defined as C =E/R where E = net voltage within the circuit and R corresponds to resistance of the connecting wire.
If the electrodes are joined together, the resistance of the circuit will be negligible leading to high flow of the current within the circuit and the magnitude of the current will be so high that it will damage the connecting wire and there are even chances of damage of battery. In order to avoid such situations, circuit breakers are used which will arrest the erratic flow of current with the breakage of the low melting wire.