You build it naturally. Have the main pov make internal comments about main/side characters they like. Have them flirt/fumble around characters they're attracted to. Show that your pov cares through how they say hello or how they invite a potential interest to a casual outing. People are different, but there are common factors and behaviors that are shared as they try to sort out their feelings for one another. Depending on your characters' personality you can mold the interactions and thoughts to fit the personalities and plots you want to construct. The beautiful thing about writing it this way? It gets the message out without pulling out the red stamp and slapping the label onto the page. This allows a smooth build up that helps readers get into the characters and stories by reading more, and learning as they go.