Though the spontaneity that you generate in this person's life is probably a big part of what attracted them to you in the beginning, ultimately, they might find you too unpredictable to really feel like this relationship is a safe bet. They don't know where the relationship is going or if they can really trust the bond you share -- heck, they might not even be able to trust you to show up when you say you will! -- and after a while this wears on them.
In some way, you're on two different pages. They wish you two could settle down a little more together; they're willing to trade in some excitement for some stability. You don't want to make that trade, however; emotional ups and downs really rev your engine, and without them, the relationship would seem boring and hardly worth the trouble. If you start to notice that you constantly seems to be running away or keeping things from getting too serious, you might need to sit down with your partner and have a talk about where each of you sees the relationship going. If you are in this for the long-term, it inevitably means that your lover, as the partner who wants more security, is going to have to compromise their emotional needs and ultimately settle for less than what they have hoped for. They shouldn't let the choice be made for them; you should talk it out and make your decision consciously, together.