This is a deeply compatible relationship. Cancer and Pisces are both water signs. In astrology water symbolizes feeling, emotion, imagination, fantasy, a sense that life is magical but lived on a feeling level. So both Pisces and Cancer are looking for the kind of love affair that's splendid, that's spiritual, that's sexual, that's very, very deep. And so they find it in each other.
One of the things that also pulls them together is that Pisces can get needy, needs someone who will be protective and nurturing, which is exactly what Cancer provides. Cancer, in astrology, is the sign of nurturing and of creating a safe and downy nest of comfort on an emotional level and creature comforts, because Cancer is also very good at making money.
So all told, we have a creative, imaginative, spiritual and at the same time very emotionally needful sign of Pisces, who fits in exactly with what a Cancer can provide. On the other side, Cancer needs the devotion and the sense of being in a relationship in which there is a deep spiritual bond, and this is exactly what Pisces can give Cancer. So altogether, they work like bread and jam.