Even though Natsume had asked about Reiner, he doesn't volunteer the same information on his own past. There was a reason he preferred this form when he wanted to relax. Basking in the sun made it harder to think or worry. He could tuck everything away - his worry, stress, emotions and unhappy memories of the past - all into a box much more easily like this, with less chance of anything spilling out.

Spotting the little grin on Reiner's face, an accusatory little claw is pointed up at him, "I see you laughing! If you're going to take a picture that's fine, but I'd like to see you try to eat like this and not make a mess!"

By which he meant, he didn't mind at all that Reiner found it amusing. He liked being able to make people smile, and he thought Reiner had a nice one.