Many Christians come from familys that teach them that marriage is important, and that part of growing up is going to school, getting married, etc.. There may also be local culture of it as well. ''All my friends are married'', and they feel left out.
Some people don't understand marriage and look to it to make them feel complete, and expect this person they marry to meet their needs, which, of course, doesn't work.
Also, i think satan uses it as a great tool to distract Christians from focusing on other things. They're so busy praying for a spouse, looking through their church, surfing the dating and chat sites that they never do anything with their lives.