Whether or not to include a love interest for the protagonist in a non-romance novel is a matter of choice for the author. Most people have at least one love interest in their lives, so my choice usually is to include some sort of love interest even if it’s just an intimate friendship.
The advantage of having a love interest is that it shows a different aspect of the protagonist’s character. One doesn’t have to dwell on personal habits and everyday hygiene, but how your protagonist treats her lover tells much about her attitude toward others. The challenge is in meshing the romantic scenes in your novel with the overall plot and protagonist’s arc. How often have we skipped through pages with the obligatory sex scene that makes no sense in the story at all.
Of course, the love interest is a major character and has to show up at least ten percent of the time. This isn’t always convenient for the protagonist, but isn’t that the way it is in real life? We’re cruising through the day and our honey calls up and asks if we’d like to go to dinner or a movie. We’re not likely to say no. Six hours later, whatever we were doing has slipped our minds.
The trick is to make the love interest an integral part of the plot, perhaps as a collaborator, or maybe as a victim. Then, when they show up, the reader knows that something is going to happen. It’s a way to insert a plot twist that doesn’t appear artificial or contrived.
In Angel of Mortality, at the third pinch point, when Raisa is faced with the new challenge of creating an antidote to the nanobot plague, she asks Damir to come visit–partly to share her misery, but also to bolster her confidence. Damir, whose goal is to marry Raisa, is disappointed that she has no interest in that, but still supports her emotionally.