In the book Night, what does Eliezer mean when he refers to his father as " his weak point"? Why has he come to view love as a weakness?

It might seem odd to refer to love and connection as a "weak point," but when you are in a survival situation, concern for anyone other than yourself, of necessity, dilutes your resources -- both physical and mental.  Thus, for example, when the selection was announced in Chapter Five, Elie had to concern himself with the question of whether or not his father would be selected, rather than simply worrying about his own chances.  Likewise, when his father appeared weak, Elie would periodically share his food rations, rather than using them to grow as strong as possible.

Yet, as Elie ultimately admits and understands, it is precisely his love and connection with his father that keeps Elie alive in the end.  During their final run from camp to camp, Elie and his father keep each other awake in the snow, and it is thoughts of wanting to stay on in support of his father that ultimately drive Elie.  Thus, while his father may be a "weak point," their connection also serves as motivation in the end.