Residents of an island nation find that coming and going are clearly defined events. Leaving or returning to an island requires more planning, and is a more clearly defined event than leaving or returning to a continental nation.
Because such coming and going both represent the transversal of a significant geographical feature, and require more time, effort, and money, island nations develop a distinct self-concept of being set apart.
Going from one continental nation to another can be done with such great ease that people are sometimes not even aware that they’ve done it.
Island nations, then, whether they are sovereign states or parts of other political entities, develop according to certain patterns produced by their geography.