grammar - Why do we invert word order when asking a question?

"Do I love you?" is the proper form. In spoken English, though, you can, and in suitable situations you would, use the form "I love you?"

To put this into an example (spoken):

Situation A:

Person A: "This guy came in and asked whether he can borrow my cell phone. Do I know him?"
Person B: "Yes, he's one of our former employees, we've worked together long time ago."

Situation B:

Person A: "Have you heard? Some guy went to the bank today and started shooting people.
Person B: "Yes, it is kind of sad, because he's one of our former employees. You two have worked together long time ago."
Person A: "I know him??"

In situation A, you are really asking the question, because you want to know the answer. In situation B, you kind of already know the answer, but for the matter of the situation, you want to assure yourself, as if to not wanting to believe the result.