We’re not suggesting you replace 'I love you' with 'I’m sorry', but think back over the past year. If you think hard enough you’ll probably remember some incident, some event or argument that could have included an apology, no matter how insignificant the mistake.
The point is not really to apologize for something your husband, wife, girlfriend, or boyfriend has already forgotten, but to use the occasion to offer a small apology as a way of saying I love you. The fact that the mistake has been forgotten is the best time to apologize for it, especially on Valentines day.
It’s just a small, added touch that can turn a simple I love you into a more meaningful illustration of that love.