Let me see if i am of any help.
Let us start first with the concept of relativistic mass. At relativistic speeds, the mass as such remains the same. But the apparent mass increases. For all practical purposes, the mass has increased. To understand this, try to visualize a universe without any losses. Next step, think on how one can increase the speed of a spaceship (easier to explain than the earth). One will use fuel of some sort to accelerate the ship to higher speeds. Now, as the velocity increases, so does the kinetic energy. The equation for the kinetic energy is given by:
K=([tex]\gamma[/tex] - 1)*m*[tex]c^2[/tex] the value of [tex]\gamma[/tex]=1/[tex]\sqrt[]{}1-v^2/c^2[/tex]
Now, the total energy is given by:
T = K + E0 E0 = m*[tex]c^2[/tex]
which gives,
T= [tex]\gamma*m*c^2[/tex]
i.e. the mass has increased [tex]\gamma[/tex] times
so u can do the calculation for the increase of the mass of the Earth using the above equation.
Now, if u can see, as the velocity is increased, the value of [tex]\gamma[/tex] increases very rapidly. This causes a very high amount of energy required to increase the kinetic energy by a small amount. That is why as the velocity increases, more energy is required for a small further increase in velocity.
As for why this phenomena occurs, i don't think relativity can help u there. :)
Hope this helps.