Children grow slowly for ten or twelve years and then, all of a sudden, they are looking down on their parents. Boys bulk up with muscle and add body hair while girls develop subcutaneous layers of fat.
Though puberty is a whole body growth process, the maturation of genitalia grabs the attention of pubertal youth because it is a relatively pleasant distraction from the rest of the growth experience.
That much physical change generates a lot of impulse activity in the nervous system and, since the brain measures pain by the number of impulses reaching it, pubertal growth tends to create considerable anxiety. To distract themselves from the worsening stress, young adults resort to behaviors they have relied on for years but, now, they feel driven beyond anything they have ever experienced before. The full force of powerful growth hormones compels them to act and at that moment, in all the world, there is just one most obvious thing to cuddle, to kiss/lick/suck, to be curious about and to rub the genitals against:
a member of the other sex.