In my life experience I have come to understand that true "heartache" refers to this unity of destiny. But don't take my word for it. Despite the fact that Keyserling is somewhat abstract in his word formulation of the problem, from the point of view of experience, we cannot avoid the conclusion that marriage creates a real unity whether we appreciate it or not.
Today, in marriage-counselling sessions we casually refer to how in marrying a person, we are not just marrying an individual -- we are marrying into a whole family.
But I submit that this is experiential proof of the unifying power of marriage. When we marry an individual we do not merely unite with their body. We embrace their entire destiny.
Nothing proves this more than a divorce involving children. As a lawyer and as a judge I have advised individuals that if they expect to escape their former spouse they are mistaken. It will never happen. For the existence of the child forever links them to the other person and to act in accordance with this knowledge is both intelligent and practical.
The actual ancient Christian wedding vow says we marry, "for better or worse, for richer or poorer, until death do us part." Many people mistakenly see this as a curse or a blessing but it is neither -- it is a description of a new state of living.
This is depicted with exquisite agony in the novel Jane Eyre by Charlotte Bronte. This is a complex novel about many subjects. But crucial to the plot are the events surrounding Jane falling in love with a man who is already married (Mr. Rochester). He is trapped in a bad marriage, one arranged for money. He is married to "Bertha", who is insane. And even though he loves another and the love is true he cannot escape the tie to his insane wife until she dies by setting the house on fire. In trying to rescue Bertha from the fire Mr. Rochester is blinded and loses a hand. He experiences despair because even though freed by Bertha's death, he fears Jane will no longer see in him the man she once loved. But fortunately he was wrong. Yet it is also clear that he could not escape the consequences of his prior marriage.
Love, Unity, and Purpose
Jane Eyre is about unity both with love and without love. Without love, there can be no purpose. Through the love of the heart, purpose is found. And if we lose that purpose the heart aches.