No one is ever unhappy because of someone else

Following a long discussion of why Stoics don't just kill themselves (after all, they have nothing to fear from losing the externals and returning to God), in which Epictetus asserts that we are to hold our station here on earth like a good soldier guarding his post because, basically, God wants us to, and because the tortures of this world aren't that bad anyway as long as we focus on virtue, Epictetus ends with this little gem:

...no one is ever unhappy because of someone else.