This is a beautiful movie, both visually with its bright colors and the soft, warm light in the scenes and in its poignant depiction of an aspect of the human condition.
The key to understanding the movie is Gerry's (Sarah Silverman) drunken speech to Margot (Michelle Williams). We all have holes in our lives, however happy we might be. We might think the "right" person would 'complete' us, but that person will inevitably create new holes.
For those unclear about the ending, Margot is with Daniel (taller, slimmer silhouette) at the end. She loves him but is sad with the understanding of all that she has lost. Not only does she no longer have the family and friends she enjoyed with Lou, she cannot replicate certain intimacies, e.g., Lou's comfort with baby talk and peeing, with Daniel.