I do not claim to be a scholar regarding the Crusades; however, what I have read leads me to several conclusions. Most of the leaders, those who wanted the Crusades, were motivated by selfish motives (power, money, land). Many of the "footsoldiers" of the Crusades had mercenary motives, as well; but there were also those who had loftier motivations and were deceived by those they followed. How terrible it must have been for those whose hearts and motives were pure to discover the selfishness and power-mongering of those around them.

There is substantial argument that Pope Urban II, who preached the First Crusade, hoped to be the Pope who re-united the Eastern and Western Churches. He had received letters from the Emperor in Constantinople asking for help against the encroaching Saracen troops.

An additional reason for the Crusade is that Urban hoped to stop the violence that was prevalent in Medieval Europe. Members of the nobility, who previously had fought against the Vikings, had no one to fight when the Vikings left, so they fought among themselves; in fact the nobility were known as bellatores, those who fight. It was so bad that the Church instituted two policies, The Truce of God which prohibited fighting on Holy Days, Sundays, and Lent, and The Peace of God which prohibited the killing of priests and other clergymen. Urban saw the Crusade as a way of channeling the pent up violent propensities of the nobility by aiming them at the professed enemies of the Church, namely Islam.

As for later Crusades, one must remember that Popes held immense political power as well as religious power. They conflicted with Emperors more than with Muslims. It is entirely possible that the Crusades were a method of extending the Pope's temporal power. As for the Crusaders themselves, they were strictly after fame and fortune. Their motto was "God wills it." but they only used that as an excuse to kill and steal, which they were already pre-disposed to do.

I don't think it was for the expansion of European empires because I do not think that there was much thought that the Holy Land was a really great prize that would make any particular country stronger.

I think that the people who went on the Crusades were motivated more by a desire for personal glory and personal riches.  The monarchs who sent the people off were motivated by a desire to get these knights and other fighters out of their kingdoms so they wouldn't make trouble at this point when there was not really any war going on in Europe.

Finally, do not discount the religious motive.  People were much more religious back in these times and there were surely many people who truly thought that it was important to take the Holy Land from the Muslims.