"The Key to My Lock"
Pair: a set of two things used together or regarded as a unit.
What's a better example of a pair than a key & its lock? Or should that be, a lock & its key?
Now a lock can be useful without a key, if it is already locked & its sole purpose is to protect whatever is behind it. But what good is the precious treasure behind the lock if the owner cannot access it? Hence the need for the lock's other half - the key.
The key is vital in making the lock truly functional & a key is really nothing useful unless it serves its purpose to open & close the lock.
What other pair has become a metaphor for so many different human situations? "You have the key to my heart" or "soul" or "mind" or "you unlocked my passion for ___, that was the key to my success." Lock & key - a perfect pair!