This is a ninety-degree square, and squares run hot. Aries and Cancer want fundamentally different things from a moment - Aries wants to charge at it, Cancer wants to feel safe inside it - and the gap between Fire and Water is where most of the friction lives. Aries's bluntness lands on Cancer like a slap; Cancer's moods read to Aries like weather it can neither control nor fix.
Who sets the course
The hidden complication is that both like to steer: put two born initiators together and, underneath the emotional mismatch, a quiet contest runs over who actually sets the course. Aries pushes openly; Cancer steers by feeling, going quiet or withdrawing when hurt - which an action-first Aries often doesn't even notice until the temperature has already dropped.
Aries and Cancer in love and friendship
In love this can be a magnetic, maddening pairing. Aries is drawn to Cancer's depth and care, Cancer to Aries's nerve and warmth, but it tends to cycle through the same wound: Aries feels smothered or guilty, Cancer feels bruised and unprotected. It works when Aries slows down enough to be gentle and Cancer learns to say the hurt out loud instead of tending it in silence. As friends the fit comes easier - Cancer's loyalty and Aries's courage complement each other well when there's no romantic pressure forcing the differences to the surface.
Beyond the sun signs
Worth remembering: a square between sun signs is a tendency, not a sentence. Two suns are maybe a thirteenth of the whole story - how the Moons meet, and where each person's Venus sits, decide far more than this one angle does. Plenty of Aries-Cancer couples thrive precisely by doing the growth the square keeps asking of them.