Isn't this whole article about success OR happiness?

Seems that way:

• The word "yes" leads to happiness.
• The word "no" leads to success.

You can't say BOTH yes and no.

Choose which you prefer.

Actually, Eric says you can find a balance:

Saying yes to everything all the time will turn you into a very happy flake who never accomplishes much.

Saying no to everything but your work will make you a miserable, lonely expert.

So how do you say yes and no?

It all starts with "protected time" for your important work.

Make a few of your prime hours inviolate. Anything threatening them gets a "no." Period.

Charlie Munger always kept one prime hour for his personal priorities.

Charlie Munger hit upon one strategy when he was a young lawyer. He decided that whenever his legal work was not as intellectually stimulating as he'd like, "I would sell the best hour of the day to myself." He would take otherwise billable time at the peak of his day and dedicate it to his own thinking and learning. "And only after improving my mind — only after I'd used my best hour improving myself — would I sell my time to my professional clients." [The Idea Hunter: How to Find the Best Ideas and Make them Happen]

For the vast majority of people this means waking up long before your first outside commitments begin.

My friend Cal Newport plans every minute of his day. You're not that much of a time management ninja? No problem.

Focus on protected days instead of protected hours.

Adam Grant has days where the door is closed, the answer is no, and important work gets done.

Other days are designated for new initiatives, helping others, and the answer is yes, yes, yes.

Read more: 

http://theweek.com/article/index/256575/the-two-words-that-will-lead-you-to-success-and-happiness