That's a fairly broad question so my answer will have to be pretty broad to. There's also differences in political theory that'll cause people to focus more on different factors. A realist for example might argue that much of the other reasons given are mere pretences or the window dressing for power politics while others would focus more, perhaps for lack of a better term, ideological and sectarian causes for conflict.

Sometimes it wouldn't really have much pretences beyond naked adventurism, norman mercenaries for example made their merry way across europe and beyond and when opportunity arose to carve out their own holdings from Ireland to Antioch.

One common one would be succession disputes. If there was any room to question or otherwise take advantage of a lack of a clear line for succession the interested parties could come to armed conflict over the matter. The classic example would be the hundred years war. At the time of the death of Charles IV he was the last living of his brothers and he had no sons. His closest male relative as such was his nephew, Edward plantagenet, who was already king Edward III of England. But he was Charles IV's nephew via Charles' sister Isabella of France who had married Edward II. Based on Salic therefor, Philip Valois claimed the title of Philip VI because although he was a more distant cousin to Charles as his Charles's uncle's son he was the closest male only line decedent. This was combined of course with other factors that had already resulted in conflicts as the considerable plantagenet holdings in France they'd acquired mostly marriage and inheritance had been previously stripped back from them by French kings, wary of the English crown having too much influence in France. Naturally bringing the two dynasties into conflict. There are other examples of such things a plenty.

Sometimes religious feuds came into play some factor in certain conflicts. I'm sure you've heard of the Crusades against Jerusalem but beyond that there were other battlegrounds. Iin Iberia after the umayyad caliphate invaded the Gothic kingdoms of the area, those conflicts continued off and on for centuries between the Christian kingdoms that would come to form in the area and the Islamic various Islamic successor states in the region that formed, fractured and reformed. Simlar stories can be told of the Balkans and southern Italy between various Islamic and Christian powers coming into conflict with religion being perhaps at least part of the story that motivates such conflicts. Then in the north between Catholic and a varity Slavic and Finno-Ugric pagan countries in Scandinavia and the Baltic the northern crusades. Plus between Christian sects against those deemed heretical such as a the Albigensian Crusade against the Carthars in France.

And many other factors that might come into play such to do with trade interests, personal animosity between rulers or within royal families, peasant revolts, conflicts between vassals becoming unhappy with a weak or tyrannical ruler.