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From A Book "Salah Ad-Din Al-Ayoobi" written by Dr. Ali Salabi. 
During this period, Western European society was dominated by local conflicts and wars between feudal princes, which helped to exacerbate the worsening social and economic situation in Western Europe. The conflicts between the two heads of the Western Christian world at that time — the pope and the emperor — also had a great effect on the course of European events. The papacy had reached a high level of power and influence during this period, which had opened the door to it becoming an international power in the sense that the pope was officially the spiritual leader of all Christians, in the East and in the West equally. However, there were ongoing conflicts between the two churches, the Eastern Orthodox and the Western Catholic, each of which insisted that its point of view should prevail and that it should have the upper hand over the other. Hence when the idea of a crusade was suggested to Pope Urban II (471-491 AH/1078-1097 CE), he saw it as a great opportunity to put an end to the dispute between the two churches and to dominate the Eastern Orthodox church and incorporate it into the Western church under his leadership, on the basis that this would all be done under the cover of fighting the Muslims, protecting the Byzantines and ‘taking back’ the holy lands in Palestine. This was in addition to many other aims that the papacy wanted to achieve through carrying out a holy war, such as putting an end to the power and influence of the feudal lords in the West and at the same time putting an end to ongoing feudal wars by diverting those energies and exploiting them in a conceited holy war that might create opportunities for petty rulers to enjoy a better life in the east without disputes.
Opinions differ as to the nature of the crusader movement and its hidden motives. Some think that the motive was materialistic; others think that it was the offspring of the zealotry and religious fanaticism that were common in Europe during the Middle Ages, and that the true motive behind these wars was in fact religious fervour mixed with other aims, such as the desire to establish new kingdoms and acquire great wealth. The majorities of historians, both classical and contemporary, regard these wars as religious wars, and believe that the religious factor was the primary motive behind them, with the aim of taking back the supposed sepulchre of Christ and the holy lands from the Muslims. Others regard that as one of the manifestations of economic and colonialist expansion in the Middle Ages. In fact the Crusades were the result of the interaction of all these factors, because they were undertaken for political, economic and social reasons, and used religion both as fuel and as a means of concealing the motives mentioned.40 It is impossible to underestimate the religious factor in these wars in any way.
Detailed discussion of the motives and causes on below will be posted 
Religious Motives
Political Motives
Social Motives
Economic Motives