The works of Fitzgerald and Hemingway each focus on love and the difficulties of love in their works. One thematic argument to make regarding the role of love in the literary work of these two writers from the period of modernism could be "the erosion of the social fabric (and values) leads to individual isolation and impossibility of true love."
Hemingway explores this concept in The Sun Also Rises, depicting the protagonist in a failed love situation with a woman who falls for a series of men (while wishing that she could be with the protagonist). Love in this novel is a romantic notion that animates the characters but cannot fully manifest itself in the real world.
Love is equally romantic and antiquated in Fitzgerald's The Great Gatsby. In this novel of moral collapse in the upper classes, relationships are marred by infidelity, deceit, and selfishness. The great romance of the novel is, in the end, an obsessive dream that fails to come to fruition.
Love has no place in the reality of this novel either.
In their other works, especially Tender is the Night and For Whom the Bell Tolls, love is depicted as complex and powerful but inevitably doomed by the greater complexities of life (social and political life). We can see this in the example of Robert Jordan, the protagonist of Hemingway's epic:
...after falling in love with Maria, who feels she has been desecrated, and joining up with Pablo, who is a fallen leader, things become complicated for [Jordan].
The complexity of love in works of modernism is in keeping with an interest in psychology; in subjective forces at work within the minds of individuals. Schisms, pathologies, and breakdowns become part of the social landscape in these works, marking relationships and isolating individuals.