Love is a tricky issue in The Left Hand of Darkness. It isn't mentioned often in the text itself, but it kind of serves as thematic glue for the entire novel. Love of others, love of one's self, love of country, love of culture: all these loves clash and interact with each other as the novel tells its tale. Perhaps the clearest example of love is the warm-feelings brewing between Ai and Estraven. Together and alone in a frozen wasteland, Ai and Estraven grow to love one another as they learn to better understand and communicate. It's a touching tale of man meets alien, falls in love with alien, and shares tent filled with unconsummated, college-dorm-room-level sexual tension. Sweet.
The fact that Ai and Estraven never enact their love sexually actually gives them a deeper bond than if they had done so.
Estraven's perception of duty is based on his love for his dead brother, Arek.