Yup, keep on having happy thoughts. Always be ready with them in times of need. Don't make the excuse that you don't have any happy thought, keep on digging. If you really can't recall a single happy moment, then it's about time you start building some, even in the simplest form. Possibly, it starts with being grateful for something.
I remember before going to the hospital for my operation, I already made a mental note of what happy thoughts I will be filling my mind. I decided to collect all my happy moments during my last travel abroad.
I also believe that my effort to keep happy thoughts (plus prayers of course) helped me recover faster.
So be conscious of what you fill your mind. No major harm in having happy thoughts most of the time during your day. It can even make you look younger, right?
Keep on smiling as well. Be thankful at all times. Make the effort.
Be happy and stay happy.
“We tend to forget that happiness doesn't come as a result of getting something we don't have, but rather of recognizing and appreciating what we do have.”