Pikachu takes a moment to think about it but then just shrugs. He's bored. That's about all the excuse he has and he knows it's really not a good one.

"Pikapika piKachu. Pi Pikapi pikaChu. (I was bored and Ash is asleep.)" Which, wouldn't normally bother Pikachu as he's more likely to just take a nap with Ash. However, they've been doing a little too much sleeping lately and Pikachu needs some kind of stimulation.