According to me

1. When Natsu threatens Hughes in Edolas that he’ll turn them all to ash if they harm Lucy in any way

2. Natsu catches Lucy when she jumps out of Phantom Lord’s tower (talk about good timing XD)
3. Virgo gives them matching outfits during the Oracion Seis arc (SO cute!)

4. Natsu invites Lucy to registrarse Fairy Tail and whisks her away as the authorities chase after them!

5. Any time Lucy hugs Natsu from behind to keep him from doing something stupid

6. Natsu and Lucy’s fight with Kain on Tenroujima, especially when Natsu is trapped under rocks and Kain almost kills her

7. Lucy clings to Natsu when Master Hades shows off his power and when Lucy catches Natsu’s scarf after he beats down Master Hades and she hugs it to her chest

8. Natsu goes to Lucy after she’s “defeated” por Flare and she says “I’m all fired up!”

9. Lucy’s freaking out about something (I can’t remember what exactly) and Natsu takes her hand (this is on Tenroujima after they defeat Hades and before Acnologia attacks)

10. Natsu freaking out while Minerva is beating up Lucy and he and Gray catch her when Minerva drops her.

I have more, but I'll write it when it gets on my mind! XD