Aries and Capricorn are both built to lead, which is exactly why the 90-degree square between them crackles. One is Fire, the other Earth; one wants the reward now, the other will wait a decade for a bigger one. Aries charges; Capricorn plans. Put them together and you get either a formidable team or a standoff, depending entirely on whether they're pointed the same way.
Two initiators, neither built to follow
That both are Cardinal signs is the heart of it - two initiators, two people accustomed to being in charge, neither temperamentally built to follow. Aries leads by impulse and force; Capricorn leads by strategy and patience. Each can find the other's method genuinely wrong: Aries thinks Capricorn is rigid, cautious, and joyless; Capricorn thinks Aries is reckless, impatient, and slightly immature.
Aries and Capricorn in love and friendship
In love the friction is real but so is the respect, and that combination can build something serious. Aries brings heat, nerve, and momentum; Capricorn brings structure, staying power, and the discipline to turn Aries's ideas into results. When they align, they're a power couple; when they don't, they butt heads over pace and control until someone yields. As friends the ambition is contagious - Capricorn steadies Aries's plans, Aries lights a fire under Capricorn's.
Beyond the sun signs
But hold the square lightly. A clash between two suns is a tendency, worth maybe a thirteenth of the true compatibility picture - the Moon-sign fit and the Mars styles, and the aspects laced across both charts decide far more than this single angle. Aries and Capricorn with softer contacts elsewhere can be a genuine force; the square just means they'll grow through some grinding on the way there.