In steve jobs' address at Stanford, why is everyone laughing after 'When I was 17, I read a quote?

Hi, I have a question. English is not my mother tongue. I'm studying now.

In steve jobs' address at Stanford, why is everyone laughing after 'When I was 17, I read a quote that went something like: If you live each day as if it was your last day, someday you’ll most certainly be right.'

I think that part is not fun or ridiculous.
Plz let me know.