Reason

Reason is the capacity for consciously making sense of things, applying logic, for establishing and verifying facts, and changing or justifying practices, institutions, and beliefs based on new or existing information. Copied from Wikipedia. 

Consciously making sense of things - I can't stop thinking of you. But I think it makes sense, when you're the first person I talk to after I woke up, the last before I sleep. 

Applying logic - We are two grown ups with mutual interest of each other, so all the times and effort we are spending for each other is completely logical. 

Establishing and Verifying facts - You were there when I was this and that and this and that, I was there too when you are this and that and that and this. Again, fact is that we are always there for each other. 

Changing or justifying practices, institutions, and beliefs based on new or existing information - So.. I think it's completely acceptable to say that I have fallen for you.