The immediate reason for launching the First Crusade was a request from the emperor of the Byzantine Empire. The Byzantine Empire had started out as the Eastern Roman Empire, which means that it was an empire based in Europe and based on the Christian faith. By the late 1000s, the Byzantine Empire was under attack from the east. Their attackers were Seljuk Turks, who were adherents of the Muslim faith. Because the Byzantines had religious and cultural links to Western Europe and the Catholic Church, and because the Seljuks were neither European nor Christian, the Byzantines felt that they could call on the West for help against the Seljuks. When this request came in, Pope Urban II called for the First Crusade.
The main condition in Europe that made the crusade possible was the fact that there were many knights and other fighting men who had little to do and not much in the way of discipline. Western Europe was at peace in these days. This meant that the fighting men did not have wars in which to participate. At the same time, the Carolingian Empire had broken down and the fighting men lacked an overarching source of discipline. This meant that there were a number of somewhat undisciplined fighting men in Western Europe who were making trouble for many of the authorities. The authorities of Europe were looking for a way to get rid of these potentially violent and dangerous people.
There were many reasons why so many people in Europe were interested in going on the First Crusade. Some people wanted to go for religious reasons. They felt that they would gain religious merit if they helped to “liberate” the Holy Land from the Muslims. Others wanted to go because they simply enjoyed fighting. The knights and other fighting men had trained their whole lives to be military men and they no longer had an outlet that allowed them to fight. Others thought that they might be able to get rich and powerful by going on Crusade. They might get plunder from places they conquered or even take territory that they could rule. People would have gone on Crusades for one or more of these reasons.