Your main story is important. Obviously it’s the most important part or you wouldn’t have a story! But no man is an island. No one exists within a vacuum. So it’s important to never neglect your B plot.
What’s a B plot I hear you say. I hope heard that, or I’m going nuts. Either way, it’s a secondary plot. While your character is on an adventure to save their village from an evil dragon, other stuff happens, too. Perhaps the rogue they met along the way has family issues and has to work something out with her father. There’s a story there, too. It supports and informs her actions and interactions with your protagonist, so it’s important to think about.
Romantic subplots are the most common kind of B plot, but don’t limit your yourself. If a character can grow and change, it’s a plot!
Keep your eyes open for my debut novel, The Paladin.