Mercury conjunct Mercury, or in harmonious signs, or Mercury conjunct the other person's sun, would be a good combination for feeling understood. All Cancers have Mercury in Cancer, Gemini, or Leo. That gives them a two out of three chance of having Mercury either in the same sign as your Mercury or the same sign as your sun. If they're a June born cancer with Mercury in Gemini, there's a good chance that their sun is conjunct your Mercury and your sun is conjunct theirs. That right there is a great aspect for a relationship, whether it's a romantic one or just a good friendship.
Harmonious Venus signs, or harmonious aspects between Venus and Mercury, also help people relate easily to each other. Most Cancers have Venus in Cancer, Gemini, or Leo, while a few have it in late Taurus or early Virgo. Leo Venus would trine your Venus by sign, and possibly by orb. Gemini Venus would sextile yours, by sign and possibly by orb, and be with your sun, too. Cancer Venus, while square your Venus by sign, would be sextile your DC sign and in the same sign as your Mercury. Venus in early Virgo would sextile your Mercury and trine your descendant, while Taurus Venus would be sextile your Mercury by sign and in your DC sign.
So, all in all, Cancers have better odds than anyone else of having both their Venus and their Mercury in harmony with yours. That's probably why you've connected with them so well.
Your descendant doesn't really indicate which sun sign would be the best partner for you. What it does is suggest what kind of traits your ideal partner would have. People with significant Cancer placements often have similar qualities to Taurus: nurturing and security-oriented.