Now. Neville/Luna, however, are another story entirely.
Luna is an outcast in her house due to being different and dreamy and her Ravenclaw classmates find her flighty and they don't think she should be one of them. They constantly tease her "Loony Lovegood", steal/hide her things, generally makes her time at Hogwarts not particularly enjoyable. In fact, she's not even mentioned until the fifth book, not even in passing.
Neville Longbottom, possible Boy-Who-Lived, outcast in Gryffindor, not generally (excepting PoA) intentional, but in his year Harry, Hermione and Ron are the Golden Trio as Seamus and Dean are both depicted as some kind of Casanova wanna-be's in most lines of plots and usually are seen around/with Parvati Patil and Lavender Brown. Which leaves... nobody to hang out with Neville.
Enter OOTP with the DA, suddenly these two outcasts are around people in a close-knit environment but are still somewhat outside the cliques. As it is, they have the chance to get to know each other better, to partner up in the DA and to generally fall haphazardly, and socially awkwardly in love with each other.
As most teenage relationships are supposed to, and do, happen.
Because that's the way it works.