This portrait of a relationship’s beginning and end has a gift for realizing and capturing the unvarnished slivers of everyday life. Ryan Gosling and Michelle Williams possess the subtle intelligence and controlled bravery to realize the two lead characters as utterly fallible human beings. The movie cuts between a day in the life of Cindy and Dean today (a time period during which all their problems come to a head) and their head-over-heels courtship six years earlier. It’s meant to contrast love at the beginning and end, but the juxtaposition actually illuminates the fact that people grow and change over time, and not always at the same pace or in the same direction. Gosling and Williams’ finely tuned detailing sustain Blue Valentine for as long as it can, even when the movie doesn’t entirely earn their commitment. The problem is that while they have clear ideas who these people are, what makes it to the screen doesn’t always articulate their relationship’s complexity. At its best, Blue Valentine is a ferociously sincere meditation on why young love doesn’t last. At worst, it’s a self-conscious variation on “Jack & Diane.”