This is a curious combination. Traditionally, these signs are considered highly incompatible due to great differences in temperament. Cancer, being moody, sensitive and somewhat secretive seem miles away from the hot-headed forcefulness and naive optimism of Aries. Yet attraction could be significant because of the mutual fascination and admiration - Cancer respects and appreciates the Arian chivalry and pride, its love-idealism. Meanwhile the Aries is seduced by the emotionality and introverted mystery of Cancer. A risk with these two is retreating together into some kind of immature indulgence in subjectivity, as if they'd wish someone would "parent" them.
But it is easy to exagerrate the differences between these signs - both are "cardinal" signs and share the action-oriented attitude to the world, making the most of opportunities. It's simply that their motives and psychology differ. While Cancer strives to fathom, possess and "mother" its near surroundings, Aries wants independence from that very same milieu, and aims for energetic progress in general. The greatest difficulty is one of understanding. Can they relate, deeply? Cancer could manipulate Aries emotionally, and Aries may hurt Cancer with their brashness and occasional tactlessness and insensitivity. But the magic is this - Aries might succeed in turning Cancer inside out, while Cancer might succeed in turning Aries outside in.