Recognizing one's soul mate depends on sensing their shared soul-root. This is the most fundamental type of insight there is.
Normally, love is born in the heart only after the initial, seminal flash of insight has, through deliberate meditation, ripened in the womb of the mind into a fully-developed idea and been assimilated into one's world view. Just as physical pregnancy requires time, so does this process, for the mentality that prevailed prior to the introduction of the new insight must come to grips with it. Since the mind and its thought-patterns are not fully refined and rectified, the introduction of a new element of truth requires that the previous mental structure be entirely re-evaluated and reconstructed in its light, and this takes time.
Exceptionally, however, one may experience love at "first sight"; love may appear simultaneously with the initial insight, without the time lapse typically required for it to develop and bear fruit. This can happen in one of two ways:
If the love is characterized by the non-self-awareness and non-self-orientation (bitul) that accompanies the flash of insight, it is barely experienced as an "emotion" in the conventional sense of the word at all. It can, in this way, shine with the brilliance of the original pristine insight. The essence of such a love-experience can be said to be contained within the mind's eye–as if the heart had "risen" into the eyes.
Alternatively, the seed of love can develop in the womb of the mind before being born in the heart, but without requiring the usual extended period of time, or in fact any time at all. Here, the emotion of love is experienced as an attribute of the heart, but one's mind is so refined that the emotion flows through it freely and naturally. The self produces no "friction" to slow the birth of the emotions from the mind.
We have likened the birth of love from the seminal insight deposited and nurtured in the womb of the mind to the conception and birth of a child, and further likened love at first sight to conceiving and giving birth without an intervening period of pregnancy. This absence of a gestation period will be the norm in the future.
In the messianic era, both the relationship between husband and wife and that between G-d and the Jewish People will be one of constant falling in love at first sight. Paradoxically, this continuous state of romantic passion will be fully integrated with the settled, stable love that characterizes seasoned relationships.
The foretaste of the world to come in this world is Shabbat ("the Sabbath") (Berachot 57b). Shabbat is essentially above the normal time-consciousness of the six days of creation. During the six days of the week (which correspond to the six emotions of the heart), reality is aware of and focused on itself. On Shabbat, the consciousness of created reality returns to its source in the Divine plan, and the emotions ascend to the level of the mind's vision. G-d re-experiences love at first sight, just as He did before creation. And since the world is continuously being created from G-d's consciousness, on Shabbat we too are able to experience love at first sight. We can both regain our inspiration with life in general, and fall in love anew with our spouses.
The essence of the Shabbat experience is thus the first level of love at first sight. The second level of love at first sight is the blessing that Shabbat bestows on the coming week.
From this it follows that when love at first sight is not experienced in the context of the holiness of Shabbat and its blessing for the coming week, it is a type of premature stillbirth. This type of love at first sight is analogous to the appearance of a premature Mashiach ("Messiah"). Our sages teach us that for the Mashiach true to come, we must keep two Sabbaths. One interpretation of this is that we need to keep one Shabbat and project its light onto the days of the coming week, thereby preparing all reality for the following Shabbat. These two Sabbaths correspond to the two manifestations of love at first sight we have described.