Saturn sits in both of their charts, and Saturn wants proof before it wants romance. It asks for consistency, for follow through, for love that shows up on a schedule rather than love that just feels big in the moment. Capricorn hears that directly, since Saturn rules the sign outright. Aquarius hears it too, but filtered through Uranus, which wants love to stay interesting, unpredictable, a little untamed even after years together. So you get two people who both distrust flimsy declarations, who both build affection slowly through action, but who disagree about what the payoff should look like once the structure is built.
Capricorn wants the relationship to function like something they can point to: a shared calendar, a five year plan, a mortgage discussed on the third date without embarrassment. Aquarius will admire that steadiness, genuinely, but will also start rearranging the furniture of the relationship just to see what happens, suggesting a spontaneous weekend trip when Capricorn has already blocked out that Saturday for something productive. That is where the friction lives. Cardinal Capricorn wants to be the one setting direction. Fixed Aquarius will not be pushed, and once it digs in, no amount of Capricorn's patient logic moves it.
What holds them together is respect rather than heat. Neither one needs constant reassurance, both take commitment seriously once given, and both would rather solve a problem together at the kitchen table than perform feelings for an audience. It works when Capricorn lets Aquarius keep one door open to the unexpected, and Aquarius lets Capricorn build something worth staying inside.