Why mass gets converted into energy.

In short, this is can be explained using Einstein's special relativity. Through a few simple thought experiments about time and space Einstein was able to show that Newtonian quantities like momentum, kinetic energy, and total energy are just low speed approximations to the true answers given by special relativity.
In fact those same quantities are.
P=gamma m v=(mv)/(1-v^2/c^2)^(1/2)
K_E=(gamma-1)mc^2
E=gamma mc^2
Now when you preform calculations using the conservation of energy or momentum you will see that mass is no longer invarient (it changes) like it was in Newtonian physics but can be transformed into energy.
For a simple example, lets say an atom of mass M decays into 2 smaller atoms of equal mass m and that they fly off in opposite directions. The masses of the two atoms would not add up to exactly the original mass. In other words m+m!= M. Some of the original mass M has been converted into the kinetic energy of the particles. By this example, the reverse is also true in that 2 perfectly inelastic colliding objects that stick together will create an object that is more massive than the sum of the original two masses (ignoring all other phenomena).
http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/relmom.html