When you love you wish to do things for. You wish to sacrifice for. You wish to serve.
Ernest Hemingway is known for his spare, “masculine” prose, and for his war novels: "A Farewell to Arms," from which this quote is taken, takes place during World War I. So his use here of the words “serve” and “sacrifice” is intentional. The line is taken from a conversation between American paramedic Lieutenant Frederic Henry and an Italian priest. The priest has asked Frederic whether he loves God; Frederic has replied, “I am afraid of him in the night sometimes.” “You should love Him,” the priest tells him, to which Frederic says, perhaps morosely, “I don’t love much.” The priest disagrees, illustrating the degree of Frederic’s love with the description above. In these ways, the priest — and perhaps Hemingway himself — seems to say, loving well is akin to serving one’s country. To love well is to be selfless.