1) "ZIVUG RISHON" AND "ZIVUG SHENI"
QUESTION: Rav Shmuel bar Rav Yitzchak says in the name of Reish Lakish that a man's Zivug is made only according to his deeds ("Lefi Ma'asav"). Rabah bar bar Chanah adds in the name of Rebbi Yochanan that the making of a Zivug is as difficult as the Splitting of the Sea (Keri'as Yam Suf).
The Gemara challenges Reish Lakish's assertion that a Zivug is determined based on a person's deeds from the statement of Rav Yehudah in the name of Rav, who says that forty days before the creation of the embryo a Bas Kol issues forth and pronounces his Zivug ("Bas Ploni l'Ploni"). If the Zivug is already determined based on Mazal, how can it be determined based on one's deeds?
The Gemara answers that only the "Zivug Sheni" is determined by one's deeds. The "Zivug Rishon" is based on Mazal.
The reason why the Zivug is determined according to one's deeds is that if a person's deeds are meritorious, he is given a better Zivug (see Rashi). Why, though, is the Zivug Rishon -- which is decreed before the person is born -- not dependent on the person's deeds? Why should a person who is a Tzadik be destined to have to marry a woman who is not a Tzadekes just because of what was decreed for him at the time of his creation?
(a) When Hash-m first created man, He created Adam and Chavah together as one, and then He separated them. The RASHBA (Teshuvos 1:60) explains that Hash-m first created man and woman together and then separated them so that they would later be able to join together and feel like a single unit. Perhaps it is for this reason that before a man is born a Bas Kol announces the selection of his Zivug. This declaration demonstrates that he and his Zivug come from the same spiritual root, and that the woman he eventually marries will be part of his own Neshamah. To bond their souls together in such a way after they are created would not be possible, and therefore Hash-m bonds them together before they are created. (If one of them is a Tzadik and the other a Rasha, the Tzadik can influence the Rasha due to their strong spiritual bond.) Of course, it is possible only for one man to be bonded to one woman. A second Zivug cannot come from the same spiritual root, and therefore the Zivug Sheni must be "Lefi Ma'asav," determined according to one's deeds.
(b) RABEINU TAM (cited by the TOSFOS SHANTZ) explains that "Zivug Sheni" refers to a widow or widower. The Zivug Sheni occurs only after the first Zivug. In order for the Zivug Sheni to occur, Hash-m must take the life of one of the spouses in the Zivug Rishon. Rabeinu Tam explains that this is the similarity between Zivug Sheni and Keri'as Yam Suf. In both of them, Hash-m must take the life of some in order to benefit others.
Rabeinu Tam may maintain that everything that occurs to a person is based on Mazal unless he is an outstanding Tzadik (as Tosfos writes in Shabbos 156a). When the Gemara says that the Zivug Rishon is pronounced before the person is born, it means that his Mazal is determined already from the time he is born. The Zivug Sheni also should be determined by his Mazal like the other major events in his life. However, there are times when a woman loses her husband not because it was the Mazal of her husband to die, but because of the great merit of another person who deserved her as his wife. This is the Zivug Sheni to which the Gemara refers.
(c) The ME'IRI takes the opposite approach. He writes that the Zivug should always be determined by the Zechus and actions of a person. When the Gemara says that the Zivug Rishon is determined by Mazal, it refers to the Zivug which a person finds when he reaches the age of Mitzvos, the proper time for getting married (see ROSH, Kuntrus Pidyon ha'Ben, end of Bechoros). Since he has not yet had the opportunity to do many Mitzvos or Aveiros, his Zivug is still determined by his Mazal. However, any spouse he finds after he has reached the age at which he is rewarded or punished for his deeds is determined according to his deeds. The Gemara calls it a "Zivug Sheni" because when a person gets married at this advanced age it is usually his second marriage (since most people, at that time, were married at the age of Bar Mitzvah).
(d) The Mekubalim explain that "Zivug Sheni" does not refer to a second marriage. Rather, it means a second matching. Hash-m determines -- before a person is born -- who will be the best match for the person. However, he receives that match only if he deserves it because of his Ma'asim Tovim. If he does not merit to receive his intended match, he ends up with another woman. This is what the Gemara calls "Zivug Sheni" (a "secondary" match in place of the primary one). (HAGAHAH in BE'ER SHEVA; YA'AVETZ; see also TASHBETZ 2:1.)