This combination is interesting for a special reason. It represents the meeting of two planetary archetypes similar to classic ideas of the male/female, as well as the two most essential "planets" in astrology - the Sun and the Moon. Cancer, ruled by the moon, flows with the tide. Her moods are just as irregular, too. Noble Leo, midday Sun, appreciates the attention they get from moon-mother Cancer, and the sense of belonging so cleverly facilitated by Cancerian imagination. Cancer is in turn warmed by Leo's drama of spirit and their generosity.
Of course, we can all see where this is going. The polarity between these signs is quite outdated, much along the lines of traditional courtship with Leo providing the male/father energies and Cancer the female/mother. In a modern relationship these stereotypes probably won't be accepted. The issue concerns how natural and smooth such a transformation of roles and expectations, if desired by the partners, can be carried out. Leo will always be enthusiastic, proud and attention-seeking. Cancer will always be in some respect touchy, caring and receptive. How these energies ultimately blend is a matter of chance or fate, I suppose.