Love seems the swiftest but it is the slowest of all growths. No man or woman really knows what perfect love is until they have been married a quarter of a century.

The remark contrasts love’s apparent immediacy—its “swift” onset as attraction or infatuation—with the long, incremental work by which enduring intimacy is actually formed. By calling love “the slowest of all growths,” the speaker frames it less as a feeling than as a cultivated capacity: patience, mutual knowledge, forgiveness, and shared history. The “quarter of a century” benchmark underscores that “perfect love” is not a youthful ideal but a mature achievement, tested by time and ordinary life. In a Twainian vein, the sentence also carries a wry corrective to romantic clichés, suggesting that what people call love early on is only a beginning, not the finished thing.