Europe in the tenth and eleventh centuries was a continent in transition. The states of Europe were still in flux and the kings of Europe had limited authority outside their own personal demesne. Although individual French kings did wield considerable power, they waged a constant struggle to have their authority recognized by the great magnates in France, especially after the fall of the Carolingian dynasty in the ninth century. The rest of Europe was no exception, in England the king was engaged in a great struggle with his leading barons and the Pope that would not be settled until the signing of the Magna Carta in 1215.
The deciding factor in the politics of the time was the feudal obligation between lord and vassal, and between king and church. The feudal relationship was in theory well defined, but in practice the greater magnates tended to act in their own interests before that of their lord. The barons of Europe were attempting to limit the authority of the king.
At the end of the eleventh century a disagreement between church and state began, in which the church asserted its primacy in matters of appointments to ecclesiastical offices. This struggle was an attempt by some clergy to reform the church from within; they contended that in order to stop clerical abuses , the church needed the sole ability to invest new clergy. This argument was best demonstrated by the struggle between Pope Gregory VII and Henry IV of Germany for control of clerical investitures, which began with a papal decree in 1075.
The Church was also trying to limit violence between Christian kings and lords. Several methods were tried including the Truce of God, which was established in 1017 as a weekend truce and later expanded on. The Crusade was seen by several Popes, notably Gregory VII, and Urban II as a way to channel the natural aggression of the nobility in a positive way.
One of the most striking differences between modern and medieval man is the respective views of religion. It is hard for a modern westerner to fathom the degree and depth of belief of the medieval European. The people of Christendom did not just believe in the death and resurrection of Jesus they felt it to the core of their being. The relics of saints and the places they lived were thought to hold some residue of their holiness. No place was more holy than Jerusalem, the place where Jesus died and was resurrected to forgive the sins of man.
Society in the medieval period was very violent; the feudal existed as a sort of protection racket where the peasantry agreed to provide sustenance for the nobility in return for military protection. Similarly, the lesser gentry agreed to serve the higher nobility in return for guarantees of protection in law and support in time of war. The entirety of medieval society had evolved for the sole purpose of supporting the mounted man-at-arms, who reigned supreme on the field of battle.
Medieval society was separated between those who pray, those who fight, those who provide. The Crusade was conceived specifically for the military, noble class of society, it was seen as an outlet for the natural militancy of the nobility, and a method whereby their military skills could be put to good use in a worthy cause. In his call to Crusade, Pope Urban II specifically called for the exclusion of the clergy and non-military classes of society from Crusade. He absolved those who could not fight from their vows, enjoining them to support the Crusade through money donations.
Lastly, Christendom was seen as being under threat from the Muslim menace, especially the brand of Islam practiced by the Seljuk Turks who had destroyed the army of Byzantium at Manzikert in 1071. There was also the matter of expelling the Muslims from the Iberian peninsula a process which began in the ninth century but was quickly seen as just as worthwhile as the Crusades in the east. In 1181, Alexius I Comnenus ascended the throne of Byzantium and appealed to the west for help against the Turkish threat. This was one reason for crusading but Urban went beyond that and called for Christian re-conquest of the Holy Land, a call with wide appeal in the west.
Europe at the end of the eleventh century was ripe for the call to Crusade, and the nobility and people of Europe saw in Crusade something bigger than themselves to which they could dedicate their life. Given the situation of the church and society, Europeans had almost no choice but to support a Crusade, the infallible Pope had called for it and it also satisfied deep longings in the hearts of the men and women of Christendom.