It was love at first sight

For whoever who doesn't know this, NaLu (NatsuxLucy) has been pretty much confirmed by Hiro Mashima (creator of Fairy Tail) a while back saying: "Lucy and Natsu's union will give birth to a child named Nashi". Hiro is saying that Natsu and Lucy will have a child named Nashi.
Why NaLu will happen:
Natsu & Lucy represent Haru and Elie who are a couple from Hiro Mashima's previous manga, Rave. With that given and the OVA when Elie said to Lucy that Haru & Natsu are their hero's then that is Hiro saying that it will happen.
Another reason is that if you check Hiro's pics on twitter, you can see Natsu & Lucy coming up a couple of times.
The last reason is that on a Fairy Tail side story (only manga) there was a time when Juvia is hallucinating and sees Natsu & Lucy doing romantic stuff.