It is hard to know for sure why so many people responded to Pope Urban’s call. We certainly have no opinion polling for the time, and the mass of the people left no diaries or letters or anything else to tell us of their motives in their own words. We are left, then, to speculate. Typically scholars talk about some mix of a desire for adventure, a desire for increased status or glory, a desire for personal gain, and a true feeling of religious fervor.
There were certainly some relatively non-idealistic reasons for people to go on what we now call the First Crusade. Younger sons of nobles might have felt that they could gain lands of their own after they conquered the Holy Land. Knights and nobles might have wanted to achieve fame and glory by fighting valiantly in this cause. People of all classes, but perhaps especially the lower classes whose lives were not often very interesting, might have felt that this would be a great adventure that would expose them to new things.
All of this said, this was a religious endeavor. This was a much more religious time than our own. People would have truly believed that they would have their sins remitted if they went on this pilgrimage/crusade. They would have believed that it was morally abhorrent to have non-believers in charge of the Holy Places of Christianity. They would have been outraged at stories of abuses of pilgrims to the Holy Land. All of this would have fired them with idealistic religious zeal.
Thus, the motives of those who went on the First Crusade were likely a mixture of idealistic and non-idealistic factors.