The word does not belong to astrology
Begin with the honest part. There is no degree, house or aspect that means soulmate, no classical author who looks for one, and no modern technique that finds one. The term came out of romantic and devotional writing and was attached to astrology by people selling readings, which is why it never quite fits the machinery.
Pisces is the sign most often sold this idea, and there is a reason. The tradition already casts it as the sign of dissolved boundaries, so writers reach for language about fusion and recognition and destiny. That is a literary decision about how to describe a water sign, not a finding about anyone born in March.
Jupiter runs it, or Neptune, depending who you ask
Classically Pisces belongs to Jupiter, which also rules Sagittarius. Modern practice gives it Neptune. The traditional reading is more informative than it sounds: Jupiter in water makes Pisces a sign of expansion in the direction of sympathy rather than in the direction of adventure, which is what distinguishes it from its fiery sibling.
Neptune, by contrast, spends roughly fourteen years in a sign, so it says essentially nothing about an individual. If the ruler of your sign is shared with everyone born in the same decade, it cannot be what makes a comparison work.
Why Cancer, Scorpio, Taurus and Capricorn
Cancer and Scorpio are the other water signs, each a third of the circle away. Taurus and Capricorn are earth, a sixth of the circle off either side. Those two separations carry a favourable classification inherited from antiquity, and the sign names are simply whatever occupies them.
The list is therefore identical for every Pisces alive, which is the plainest possible sign that it is describing a wheel rather than a person.
The comparison that has something in it
Two suns against each other is a fraction of the material. A comparison with content reads both Moons for what each person needs when unobserved, both Venus placements for how fondness is given and read, both Mars placements for how a disagreement moves, and both ascendants for what each presents first. It then measures the angles running from one chart to the other, which is synastry, and the synastry calculator does the measuring from two birth times.
Mutability is worth adding for Pisces specifically: two mutable people can accommodate one another indefinitely without either ever stating a preference, which is a real difficulty and one no shortlist flags.
Read the pair, not the position
Take the combination you are actually curious about. Pisces and Gemini, a mutable square, is a sharper read than Pisces and Cancer, because it has to explain how two flexible people manage to frustrate each other.
Then compare the charts if both birth times are known. Smaller question, real answer.
Questions people ask about this
Which signs are named as good matches for Pisces?
Cancer and Scorpio in water, Taurus and Capricorn in earth. The list is generated by the angles, not by observation.
Why is Pisces so often linked to soulmates?
Because the tradition describes it in terms of dissolved boundaries, which invites that language. It is a stylistic habit, not a technique.
What rules Pisces?
Jupiter traditionally, Neptune in modern practice. Neptune spends about fourteen years in a sign, so it says little about an individual.
What is worth comparing instead of two suns?
Both Moons, Venus, Mars and ascendants, and the aspects between the two charts.