Do you laid-back people ever become stressed? if so, do to what?

I am interested currently, so i started researching online about laid back persona's, and all that. And i realized that I caused one laid back person to be stressed, upset, and unhappy for a while. however, since then, he was never unhappy again, and was always happy. And it was a guy (regarding relationship stuff). So. What are some things, or people/issues that makes you unhappy? IF* you are someone who is usually calm?