Little children often have a lot of other kids to play with but when they get older and start pairing off, anyone who does not have someone may have a great deal of time which is not filled with activity. Just sitting around much of the time, aware of what used to be obscured by excitement, can be a pretty raw feeling which someone compared to acid on skin. It is referred to as "loneliness."
Being alone much of the time is a major motivator which drives people to look for someone to be with. The longer that one is unsuccessful, the greater the motivation until, in some cases, frustration turns to depression and/or anger and/or even greater extremes including sex crimes and suicide in variously unstable individuals.
Loneliness becomes added to the other factors which drive confused members of the so-called "hormonal set" to seek out the distractions that they learned, early in life, to use to reduce the stresses that humans feel. It is one more added pressure but, because it happens in the same individual(s) at the same time as all the other factors, it seems like part of one overwhelming force: the long misunderstood and mislabeled "sex drive."