Partners may grow closer or drift apart as time passes. The partnerships are not idealized. Partners shift and change, too. Sometimes new partnerships work, sometimes they don't. They grow, they evolve.
Partners spend so much time together, they know each other well.
They don't choose their partner, usually, they're thrown together. Sometimes there's magic in these pairings. Frank and Tim. Meldrick and Crosetti. Kay and Beau.