Alcott sends Mr. March away to the Civil War. This choice allows readers to see the relationships among the March sisters and Marmee without the overshadowing presence of a man in the home, a dynamic that is arguably more interesting than the dynamic presented when he returns home. This also adds some suspense because of the danger posed to Mr. March, and thus the welfare of the family. His injuries also take Marmee away for some time, giving the girls a chance to really prove their maturity, especially when dealing with Beth's illness. Her scarlet fever, contracted when she was helping the poor Hummel family, as well as the passing of their baby, also help to heighten tension in the novel and to hold readers' interest.
The love story, of course, between Meg and Mr. Brooke, as well as Laurie's proposal to Jo, help to increase interest in the text. We've become invested in these characters' feelings, especially Jo's and Laurie's, and though we can see her rejection of him coming, he cannot. His proposal, followed by his European exploits, increases interest because readers are left to wonder, for some time, whether Jo's rejection of Laurie will change him for the worse forever. Romances and unrequited love are elements of plot that are often used to retain reader or viewer interest.