The relationship should start with some sort of interaction that is not love at first sight. For example, the dragon might kidnap the princess for a reason that is not related to love, like to use her as a bargaining chip. But during their time together, they start to understand each other's motives and personalities. This can lead to a gradual development of feelings.
In a 'princess and dragon love fanfic', the relationship can develop realistically through shared experiences. They could go on adventures together, facing dangerous creatures and difficult terrains. Through these shared trials, they learn about each other's strengths and weaknesses. The princess might see the dragon's vulnerability and the dragon might admire the princess's courage. This mutual admiration and understanding can then turn into love over time. For instance, when they are trapped in a magical maze, the dragon uses his fire to keep them warm and the princess uses her wits to find the exit. These moments of relying on each other can deepen their connection.
The development can be based on mutual needs. Say the princess is in need of protection from an evil force, and the dragon is in need of someone to break a spell on his lair. They make a deal. As they work towards their respective goals, they get to know each other better. The dragon shows his loyalty and the princess her kindness, and love grows from there. It's a slow process of building trust and respect.