Relationships should be all about progress -- your own, personal growth that comes from interacting with another person and learning to compromise and cooperate; and also, the growth of the relationship itself. To stagnate -- getting stuck in one place, or running around in circles -- isn't fun, and it isn't the ideal, natural human condition. But you have a tendency in love affairs to do just that: to run around in circles, chasing your own tail, instead of learning your lesson and moving forward (or, if the case may be, moving on).
You might stay in a relationship a lot longer than you should, but it's more likely that you'll move on prematurely, without letting things deepen. Deep down, the idea of moving from the attraction phase to the commitment phase scares you, because you know all that other stuff will necessarily come along with it -- uncomfortable negotiations, compromises that aren't as fun as having things just how you want them, when you want them. But what's the cost? Refusing to progress in a relationship or within yourself keeps you running around in circles, never learning anything new about yourself, about love or about that special someone.