How come the Medieval Ages are full of wars? Were the rulers during that time really as power-hungry are described?
Or was because of religion? In fact, what religion have to do with it? What made many leaders or rulers think that they obligated to make wars and conquer lands because of religious beliefs?
I think you are making a conceptual error in thinking that the Middle Ages were more violent than any other given time period.
People have fought over territory, resources, religion, and political control for as long as we have historical records. Biologists have seen chimpanzee bands fight over territory, and fossils of early humans have been found with wounds that appear to be inflicted by other humans, so it is entirely possible that humans have fought wars for longer than we have been humans (depending on how you want to define war and human).
As u/babelfiish pointed out the premise that the middle ages were any more violent than other periods in history is false, just looking at the past hundred years disproves this. Human history is full of conflict.
Is your question about why wars were fought in the middle ages? This is exceedingly general and covers an enormous period of time (roughly 500 ad to 1500 ad) during which many wars were fought for a great range of reasons (religion, trade, land, succession and previous wars, just to name only a few of the causes).
We can at best attempt to look at individual cases, but there is no single root cause for all of human conflict during any period.