Don't interrupt - then why?

Yesterday morning we were enjoying the usual weekend cappuccino. Lorenzo didn't want to sit with us, and went playing with his xilophone. It was a present of his uncle, a proper instrument, and he much prefers it to the wooden one, since it's way more noisy :-)
He can't open it (it's difficult for me as well) and always ends up frustrated in the attempt. Not yesterday. Yesterday he was happily playing with its retractable legs. So I stood up from the table, I went over, and I opened it for him.
He started his music production immediately obviously, but I wondered - why did I do it?
My intervention was uncalled for and unneeded.
The overly passionate "don't-interrupt!"-advocate (me) anticipated his desire to play and his frustration but, in reality, interrupted his real activity in that moment.
It's not easy to see clearly sometimes!