Attentiveness describes the act of paying attention to an object; to be "attentive" is to be engaged in the act of paying attention; "attention" is defined as "the act or faculty of attending" [0], faculty being "an ability, natural or acquired, for a particular kind of action" [0]. Since we all have this ability naturally rather than having to acquire it, as is apparent simply by noticing that you're sitting reading this post right now, it's just a case of recognizing this "particular kind of action" occurring in real-time and learning how to utilize it effectively.