The Jewish tradition that claims that marriages are arranged in heaven is very old and a very romantic idea. It first appeared in the Talmud: "Rab Yehudah said in the name of Rab: Forty days before the creation of a child, a divine voice (bat kol) issues forth and proclaims in heaven, 'The daughter of A is for B!'" (Sotah 2a) Later in the Zohar, the classic work of Kabbalah, this idea found its full expression: "Each soul, prior to entering this world consists of a male and female united in one being. When it descends on this earth, the two parts separate and animate two different bodies. At the time of marriage, the Holy One who knows all souls and spirits, unites them as they were before and again they constitute one body and one soul…" (Zohar 1:91a) I believe that this is the basis of the term bashert, that every person has one person who is their destined partner in life.
If we were to understand this idea literally then you are asking a valid question. It might seem cruel for god to create our destined partner, a soul mate, for us and then to separate us from them so that we have to spend a life time searching for the person with whom we are supposed to be. Of course, there is something to be said for having to labor at accomplishing significant things in life. Searching for a treasure makes the treasure even more precious. I am not sure we would value our soul mate as much if we didn’t have to go out into the world and search for them ourselves. That search also allows us to learn about ourselves, our likes and dislikes , and our priorities. Remember that the Talmud also tells us that while God may make matches, match making is as hard as "splitting the Red Sea." (Bereshit Rabbah 68:4) Even if someone is our soul mate does not mean that it is enough to simply be matched up with them.
Personally, I do not understand this idea quite so literally. We cannot know God's ways or the mysteries of the universe. Love itself is also a mystery. Why do two people fall in love with one another (and why do they sometimes fall out of love, if they were destined for one another)? What is it that draws them together? We can’t know the answers to these questions.
Yet when we fall in love and find the person with whom we wish to spend our lives, we feel such a deep sense of connection that our love seems to transcend the mortal world. That is why people in love often describe that person as their "bashert," as their soul mate, with whom they were destined to be. Once they meet they cannot imagine their life without them. That is nothing less than a miracle - and surely, in some way God must be responsible for bringing these two people together and inspiring a love that transcends self interest. In counseling couples before marriage and speaking to seasoned couples who are still very much in love after decades of marriage, they often describe their relationship as two halves of a single soul. So maybe this idea of being separated from ones partner only to find them again in the world is not so much a description of what happens but what it means to be completely in love with another person. This is not a statement of what is, but a description of what we strive for in a marriage.
Rabbi Mark Greenspan