This type of relationship is strictly physical. It is the attraction that two people share from the first time meeting each other, and they float for a while on only that attraction, realizing later that they do not have any true respect for each other. Rilke comments, "they notice with astonishment and indignation only from the dissension that arises between them out of all this disorder." Yet the two people for sake of a relationship hold on to that single physical attraction, what they thought was love.
Finally a third cause for a young people's mistake in their relationships is the social context which says that people are not happy when they are alone. When someone is single a general social feeling is that they must be lonely. We see it in movies all the time, for example movies like "Sleepless in Seattle," or "When Harry Met Sally," (basically every Meg Ryan movie). The movies show that a single person, even if their life seems to be going all right is secretly seeking relationships. Young people do not want to be that older miserable lonely person who is desperate for love. So, they jump into a bad relationship in the hope of a long lasting love.
In time the youth will become older and they realize the error in their ways. They often forget or overlook what they once found attractive about their partner, or whatever caused their relationship. Therefore, whatever caused the relationship is now dead. Rilke says, "In his uncertainty each becomes more and more unjust to the other; they who wanted to do each other good are now handling each other in an imperious and intolerant manner." He means that the two individuals were blindly holding on to something stagnate and now realize that movement needs to occur, resulting in either a change in the relationship, or the demise of it.
Young people can fall in love, and according to Rilke they have to learn it. He compares young people to being apprentices of love.