There's a very crude joke to be made here.
[Nothing. He has done absolutely nothing to deserve this. This is completely unearned suffocation.
Naturally, Ciel's laughter makes Sebastian snap out of it and realise how ridiculous he's bei... Or, you know, all Sebastian is aware of is that this is suddenly the best day of his life and he's so happy and he loves this kitten with all his heart and kittens have such awesomely sharp claws despite being so small. You can practically see the little hearts flying out of him.
He mumbles something. It's hard to make out what he's saying into the cat's fur, but it's probably not something particularly intelligent (for the record, he's trying to say something like "you are so lovely and I am so honoured to meet you, but may I please hold you in a way that's more comfortable for you..."). Possibly the kitten relaxes somewhat, because of Ciel's laughter?