You two just don't understand each other's ideals. Your dreams don't match up; you have two different visions of what life's all about and what's your unique place in it. When you try to talk these things over, you might not even understand what the other's saying, because you're on two such different planes of thought and perception.
This could even manifest in a very literal way -- one of you always remembers your dreams when you wake up in the morning, but the other doesn't; one of you has lots of ideals about your courtship or what love is supposed to feel like, and the other thinks it's unimportant, or just up to random chance. Or this could occur on a more subtle but important level: The ideals each of you holds in your own mind for your future just don't match up. One of you wants a family, while the other imagines being a loner all their life. This kind of mismatch makes it hard to feel connected on a very deep level; even if things are okay on the surface, and you're getting along and having a grand romance, you both feel the disconnect underneath.