As they travel, their bond deepens. They start to rely on each other more. Natsu becomes more protective of Lucy, and Lucy begins to understand Natsu's impulsive nature better.
Well, Natsu is a passionate character. He could react with a lot of excitement. He might pick Lucy up and spin her around, laughing and saying how amazing this is. Or, he could also get a bit worried and start asking Lucy all sorts of questions about her health and what they need to do next.
Well, in some fanfictions, it could be through a dangerous situation. Maybe Lucy is in peril and Natsu's intense fear of losing her makes him realize his love. For example, if she's captured by a powerful enemy, Natsu's desperation to save her and the way his heart races at the thought of her being hurt could be the trigger.
Lucy might be heartbroken. After all, Team Natsu was like a family to her. She could initially go into a state of shock and disbelief. Maybe she would try to plead with the team to let her back in, explaining that whatever they thought she did was a misunderstanding.
Perhaps in the fanfiction's plot, Natsu makes a big mistake that endangers Lucy or her friends. Let's say he recklessly takes on a powerful enemy without a proper plan, and Lucy's friends get hurt as a result. This might cause her to hate Natsu for his irresponsible behavior.
In many fanfictions, Natsu protects Lucy by using his powerful fire magic against their enemies. For example, if they encounter a dark guild attacking Lucy, Natsu would jump in front of her and unleash a flurry of fire attacks like his 'Fire Dragon's Roar' to drive the enemies away.
There are fanfictions where Natsu's confession is more of a spur - of - the - moment thing during an argument. Maybe they are fighting about something stupid, and in the heat of the moment, Natsu blurts out his feelings. He might say something like 'I can't stand this arguing with you, Lucy, because I love you too much.'
In some fanfictions, Lucy might use her Celestial Spirit magic. For example, she could summon a powerful spirit like Leo to help fight off whatever is threatening Natsu. The spirit could provide a distraction or directly combat the enemy while Lucy tries to reach Natsu and get him to safety.