Well the answer is:
Biological: Human being as such is a sexual object. This may seem hard to digest but ask any biologist and he will tell you that primary aim of all living beings is to procreate and maintain their population dynamics on earth. Since humans are single sexed animals it is necessary for male and female of the species to come together and copulate to in order to facilitate reproduction.
Thus chemistry within us works as such that we have a strong desire to mate and hence when humans reach sexual maturity they get pre occupied with the opposite sex and try to meet and interact. The first option is meeting at the place of work, school, and college and of course online dating sites.
In traditional Communities especially in the Orient it is a norm for parents to help their siblings find a mate but this usually is for marriage and young adult rarely get together before marriage and live a segregated life till the nuptials.
But as in present time whatever the West does East follows. Hence dating is catching up in these traditional communities fast and teens are dating more and more now for future matchmaking or romance. Whereas adults go for sex dating to make up for a lack of permanent partner or for their preference for casual adult dating. Some also wants Gay dating and lesbian dating.
Online dating and speed dating are among the most popular ways singles date today. Online or cyber dating is done by placing online ads in web sites that provide dating services, messaging-e-mails, chatting and live web cams.
Emotional Need: Here it is more a feeling insecurity among humans who live their lives as singles or who are forced to live their lives as singles for some reason or others. The desire to share responsibility and that get rid of terrible feeling of loneliness is another reason why male and female come to live together. All this makes sense as any body would tell you. Thus marriage is a major event in our lives. Though humans have now started to live together in a non conditional or a relationship devoid of any conjugal ties.