Start with two actual charts
An Aries born at dawn with Mars in Capricorn and the Moon in Pisces behaves nothing like an Aries born the same afternoon with Mars in Aries and the Moon in Sagittarius. Same sun, same birthday, two temperaments that would not recognise each other. Any list that treats them as interchangeable has already lost the thread.
This matters more for Aries than for most signs, because Mars rules Aries and Mars is the fastest-moving of the traditional rulers after the Moon and Mercury. The sign it occupies changes roughly every six weeks, so two people who share the Aries sun frequently do not share the planet that runs it.
Who the tradition names, and on what grounds
Leo and Sagittarius are the other fire signs. Gemini and Aquarius are air, and air is said to feed fire. Measured on the wheel, Leo and Sagittarius each stand exactly a third of the circle from Aries, and Gemini and Aquarius each stand a sixth of it away. Those two distances, 120 and 60 degrees, are the ones classical astrology filed under easy.
So the recommendation is arithmetic wearing a personality. Nobody counted Aries marriages and found the fire ones lasting longer; the angle was ranked benign centuries ago and the sign names were read off it afterwards.
What Mars is doing in the argument
Mars is appetite and the willingness to start something before the ground is clear. Aries wants a decision made and then left alone, and the objection to the traditional shortlist is that it does not test for this at all. A Leo with Mars in Taurus will dig in over exactly the thing an Aries wanted settled by Tuesday.
A comparison that earns the name puts both Mars placements side by side and asks what a row between them looks like, then adds the Moon for private need, Venus for how warmth is offered, and the rising sign for first impressions. Comparing the aspects that run between two charts, rather than inside one, is synastry, and the synastry calculator will do the measuring from two birth times.
The word itself
Soulmate is not an astrological term. No degree, house or aspect stands for it, and the older texts do not reach for the idea at all. Writers who use it have imported it from somewhere else and left the astrology to carry a claim it never made.
Aries has a particular vulnerability here, which is that speed feels like certainty. The rush at the start of something is real information about chemistry and no information at all about duration.
A better use of an afternoon
Pick the sign you are genuinely wondering about and read that pairing in full, whether or not it made the shortlist. Aries and Capricorn is a more instructive read than Aries and Sagittarius, precisely because the friction is described rather than waved through.
Then, if the birth times exist, put the charts against each other. That comparison answers a smaller question than the one people arrive with, and it is the only part of the exercise doing any work.
Questions people ask about this
Which signs are said to suit Aries best?
Leo and Sagittarius by shared element, Gemini and Aquarius at sixty degrees. The list is a by-product of the angles rather than a finding about people.
Does a birth chart show who your soulmate is?
No. There is no placement that carries that meaning, and the tradition has never claimed one. A comparison describes tendencies.
Why does Mars matter so much for Aries?
Mars rules the sign and moves through a new sign roughly every six weeks, so two Aries born weeks apart can differ sharply in temper and drive.
What is worth reading instead of the sun sign?
The Moon, Venus, Mars and the ascendant in both charts, and the aspects between the two.