Every couple is not a pair.

The proverb distinguishes a couple, understood as two brought together, from a pair whose members fit each other. Mere association does not establish compatibility.

It challenges the assumption that a partnership or grouping works simply because it consists of two. The quality of the match matters more than the fact of being joined.

Use it when someone treats two people or things as a fitting match solely because they have been placed together. It can express doubt about how well a proposed partnership or grouping will work.