Although they're deeply in love, conflict interrupts their plans to be together, and the lovers part.
Time passes, often several years.
Both build separate lives, careers, maybe even other marriages, but neither is ever really happy. When they meet again, their earlier conflict seems to have eased away. They try to get together again, but the conflict may be real, or their newly found happiness, always tinged with regret, may be threatened by other factors. Love and devotion are considered more or less as ideals, but in a romance with this theme, the ideal is subject to the frailties of human personality. Family objections, differences in background, intricacies of other relationships, any number of overwhelming forces separate the lovers.