Air on fire
Air feeds fire, and the mechanism is literal enough to be useful. Aquarius supplies the idea, the framing, the reason a thing is worth doing at all, and Aries supplies the willingness to begin it that afternoon. Neither element drains the other, so the pairing tends to be energising rather than tiring.
The cost is that nothing in the combination is cautious. Air abstracts and fire commits, and between them there is no faculty for checking whether the plan survives contact with money, time or other people. Couples in this pairing often report the same pattern: excellent at starting, indifferent at maintaining.
Fixed against cardinal
Modality is the more revealing half of this. Aries is cardinal and initiates constantly; Aquarius is fixed and does not revise a settled view. Because their subjects rarely overlap, the clash is delayed rather than avoided.
When it arrives it concerns direction. Aries wants to redirect the whole enterprise on the strength of a new enthusiasm, and Aquarius, having already reasoned its way to a conclusion, treats the redirection as noise. Aries can be argued into things; Aquarius cannot, because Aquarius arrived by argument in the first place and considers the matter closed.
Mars, Uranus and Saturn
Aries is ruled by Mars. Aquarius is ruled by Uranus in modern practice and by Saturn in the older scheme, and both readings pay here. Saturn explains why an Aquarius position hardens; Uranus explains why the position was unorthodox to begin with.
Mars alongside Uranus is a fast, unstable combination that produces sudden decisions on both sides. Mars alongside Saturn is slower and more grinding. Which of the two you are actually living with depends on the condition of those bodies in the two charts, not on the sun signs.
Where it grinds
Feeling. Aries expresses emotion at full volume and considers it discharged; Aquarius steps back from emotion in order to examine it. Aries reads that step back as coldness, and it is not always wrong.
The second problem is company. Aquarius belongs to groups and brings the relationship into them. Aries wants attention directed at it specifically, and a social life organised around causes and collectives supplies very little of that.
Why the sun signs settle nothing
A sun-sign match describes an average drawn from an enormous number of people, and no couple is an average. The Moon determines what each of them needs when tired, Venus determines how affection is offered, Mars determines pace and conflict.
Two pairs of Aquarius and Aries can look nothing alike once those are on the table. The sun comparison gives you a plausible sketch of the temperaments and no information at all about the two people reading it.
Questions people ask about this
Do Aquarius and Aries get on?
Usually easily at first, because air and fire reinforce each other and neither sign requires much reassurance. Longer-term the question is whether anyone in the pairing handles maintenance.
Which sign is more stubborn?
Aquarius, and by a distance. Aries holds a position loudly and briefly. Aquarius holds one quietly and indefinitely, having reasoned its way there.
Is Aquarius ruled by Uranus or Saturn?
Modern practice says Uranus, traditional practice says Saturn. Reading both is worth the effort, since they account for different parts of the behaviour.
What matters more than the sun signs?
The Moon, Venus and Mars in both charts, and the aspects between them. Those describe need, affection and pace, which is most of what daily life consists of.