In general, knights in the old days were like soldiers are now. What I mean is that they did not really choose when they would be sent to war. Instead, they would go and fight when they were told to go.
For knights, they owed allegiance, or loyalty, to a lord -- someone whom they had to obey. When their lord decided to go war (on when the lord's lord decided to go to war) the knights would have to fight.
So knights didn't choose whether to fight or not. They were soldiers who fought when they were told to fight.
There are many reasons WHY a knight fought. Some fought for honor and truly tried to uphold a code of chivalry while others went to war to gain prestige and for profit, El Cid is a wonderful example of the latter. While the remainder did so for the sake of self-preservation, these knights were often very poor and if they chose not to fight for tyhe lord or local noble calling them to arms the man would be thrown of the lords manor and thus his wife and children would be starved out soon after.To understand the more honorable of those we must examine what it took to become a knight in Medieval Europe and understand that these are general guidelines not hard and fast rules. Becoming a Knight in Medieval Europe was much like becoming a buddist monk in Southeast Asia is today. First, you had to be born into the noble class. Second, a boy would begin training at a very young age probably 6-8 years old as a page, his duties would be caring for his knights horses, weapons and armor. The around 12-15 years old if the boy had proven both his loyalty and skill to the knight he could become a squire, he would then learn combat skills and how to dawn the armor as well as the various tasks above, the squire would also be responsible for teaching new pages their duties. After several years of training as a squire if the young man had proven himself yet again to the knight and the local lord through obedience, and valor in combat he could then become a knight and begin the cycle for himself, or he could choose to continue serving under the knight with which he had trained as a Vassel Knight. There are a few cases in which a commoner became a knight simply through a display of valor in combat, William Wallace is an example of such
Knights were hired by lords and kings to help protect their land and property. In exchange, lords and kings would give them something in return. It might have been land, a castle, or whatever the knight might request. Most knights were around 18 years old when they were first "knighted" by a lord or king. They had to prove their skills, and "chivalry;" which meant that they had to pass a series of tests that showed that they knew how to ride a horse and fight. They also had to fast (not eat for a certain amount of time), and swear loyalty to the lord or king.
A lot of the knights who fought in wars and battles did not live past 21 years old. Many knights also fought in the Crusades. The Crusades were a long war that was started by Pope Urban II. He was in charge of the Catholic Church. The pope felt that God wanted the Christians to take back the Holy Land in Jerusalem. At the time, it was controlled by the Muslims. Many knights volunteered to fight for Christianity. The knights won some battles, and lost some battles. In the end, they did not succeed. Many lives were lost. When guns and cannons were invented, knights were no longer needed, and so they sort of "went out of business."
because they wanted to help save each other lifes, and fight the enemies that they did not like. They wanted to save their lifes and, the people that were knights. They didn't want their partners to die or get killed, so they had to save their people that they were fighting for.