What is the reason that light emitted by the stars should be able to keep on travelling without any time limit? Water flows from a higher level toward lower level but finally stops somewhere. Electricity also flows from higher potential toward lower potential but stops somewhere unlike light. Likewise, light flows from a higher level toward a lower level of "what" that it can keep on travelling in space endlessly with no "time limit" whatsoever as long as it is not obstructed on its way by something?  
 A: This isn't a property that's unique to light. Any object can travel through space endlessly, as long as it's not obstructed by anything. For the terrestrial examples you mentioned, there's a physical reason why the system stops changing. Water flows downhill until it's stopped by something that prevents it from flowing further. Out in the far reaches of space, there's (very close to) zero forces acting that would stop an object already in motion. It's not that these objects are flowing "downhill" toward a lower energy state, they're just coasting without anything to stop them.
A: 
as long as it is not obstructed on its way by something

Light as classical electromagnetic waves, is an emergent phenomenon from the confluence/superposition  of innumerable photons that compose it.
Light, from a star for example , will be radiating energy falling as $1/r^2$ per meter square as it disperses. If there is absolutely no obstruction, at very large distances, the number of photons per meter square will become so small that no "visible  light" superposition of photons can be perceived by the eye. Photons, as elementary particles, will continue forever, if there is no obstruction.
Now both a large number of photons, as light, and individual photons may meet obstacles. At the photon level it is elementary scatterings that may absorb photons or deflect them. At light level, electromagnetic wave, it may also be absorbed by dust clouds etc, its intensity diminishing.
In the real universe, an additional interaction is the expansion of space, as in the Big Bang model, where the energy of the photons is doppler shifted , and thus can fall far into the infrared and microwave frequencies. 
The cosmic microwave background radiation we are measuring started at high energies , in the current Big  Bang Model of the universe, at the time of decoupling of the photons, and is now very very low energy.  So in our universe, yes, unobstructed light does not disappear, it becomes less and less energetic per square meter ,with time.
When, because of the dispersion, there is no longer a superposition but just individual photons, light is no more, so light does not go on forever. Photons do unless they interact.
The end of the universe would stop the photons in some way, various models are proposed for the end of the universe, an the photons fate will depend on which is the model nature will follow.
A: Because it was not absorbed? The real reason was such that electric magnetic field carried energy and Energy Was Conserved. In the form that the electric field and magnetic field oscillates together and travel in vacuum space.
A: Adding to Nuclear's answer, the concept of potential comes up when there is a force around (a conservative force to be precise). Water flows from high to low altitude because it is pulled by a force called gravity. Electrons from high potential to low potential because they are experiencing a force called electrostatic force.
Light on the other hand is neither made of matter (like water or electrons) nor does it experience any sort of force. It is more like sound waves, which weaken as they spread but can go on infinitely (that is to say if there is no loss of energy, which, in light's case, is a reasonable assumption to make).
A: The light carries energy. Conservation of energy says that energy will remain in existence until something happens to change it to some other kind of energy.

Water flows from a higher level toward lower level but finally stops somewhere. ... Likewise, light flows from a higher level toward a lower level of "what"

Light doesn't flow preferentially in any particular direction. It just radiates away from whatever source produces it (unless it interacts with some material like a prism or mirror that can change its direction). It's entirely possible for the Sun to be radiating light towards Alpha Centauri, while simultaneously Alpha Centauri radiates light back in exactly the opposite direction toward the Sun.
