As far as Gajeel was concerned everything was Natsu's fault. All of it. Every single little event that had happened from the moment he had woken up on Sunday morning (which was his day off by the way) until now was Natsu's fault.
It was Natsu's fault he had a Girlfriend (well, truthfully it was the Girlfriend's fault, because Natsu? Seriously? Although Natsu must have had something to do with it). It was Natsu's fault he was an idiot (well, maybe that could be attributed to his moron father Igneel, but Gajeel would rather blame Natsu's idiocy on Natsu). It was Natsu's fault he had decided to cook breakfast for his Girlfriend (the dumbass couldn't even make cereal and his bimbo Girlfriend should have known that). It was Natsu's fault that he burned down her and her Roommate's place (and the Girlfriend's fault for having a Roommate). Gajeel could probably find a way for it to be Natsu's fault that Natsu's Girlfriend's Roommate was hot; well it was Natsu's fault that he chose a Girlfriend with a hot Roommate so he guessed that counted.
Not to mention it was definitely Natsu's fault that the Roommate and Natsu's Girlfriend (whose name was Lucy by the way) were now staying at Gajeel and Natsu's place until they found a new one, and since he invited them in public and with that Roommate's beautiful blue hair and small cute face and her round hopeful eyes, Gajeel couldn't say no. It was Natsu's fault that (since the two didn't have a guest room) the Roommate was sleeping on the couch right now. It was also Natsu's fault that Gajeel (who woke up every day at 4 to get to the restaurant on time) would have to see her (he was not watching her damnit! It was normal to look around a room when you walked into one) sleeping (she was so angelic by the way, sweet and childlike, her hair splayed around her face and a small line of drool creating a puddle on the pillow).
So, by extension, it was Natsu's fault that the Roommate was now cooking in Gajeel's kitchen (she'd wanted to cook because Gajeel was always cooking—he was a chef, what did one expect him to do?—and she wanted to give him a home-cooked meal without him having to make it). It was (unsurprisingly) also Natsu's fault that Gajeel never let anyone else in his kitchen (because the only other person in his house was Natsu {most of the time}, and Natsu was never under any circumstances allowed in the kitchen—the proof was now living in his house; sleeping in his cousin's bed {well they were dating} and on his couch {with her cute little face and her soft silky blue hair…}). But the fact that Gajeel was letting the Roommate (her name was Levy by the way) cook in his kitchen meant only one thing.
That Gajeel was falling in love with Levy McGarden, the Roommate.
And, because everything else was his doing, that was Natsu's fault also.