The main reason is movies. i like how they ship neville and luna in the movies.

Harry Potter:Ginny! Neville! Are you all right?

Neville Longbottom:Never better! I feel like I could spit fire! You haven't seen Luna have you?

Harry Potter:Luna?

Neville Longbottom:I'm mad for her. I think it's about time I told her, since we'll probably both be dead by dawn!

And after the battle of hogwarts: