This topic is something that I believe can go either way depending on your opinion. I tend to agree with one side at certain times, and switch back and fourth. Though these are my thoughts of both.
Taking on the debate from the feeling perspective, you can not be happy all the time. You are put in situations that can make you feel happy. Even if things happen that make you sad or upset, for example, a funeral, you can think about the good memories that you have had with that person, making you feel for a moment a surge of happiness. The feeling of happiness can come along without you deciding to do so. It can just pop up on you when you least expect it to. Different things make different people happy because we are all individuals. So with that state of mind, you can not make yourself happy by saying today's the day. You have to do, hear, see, and smell something in order for the feeling of happiness to come along with it.
For those who think it's a choice, I lean more towards this side as well. In my past, if I woke up and said to myself that today was going to be as bad as the one before it then obviously it was going to be. You have to create a mindset that you want to be happy. If you allow yourself the chance to let happiness be a part of your life, then you will ultimately be happy in many of the things that you do.
Happiness may be an overwhelming feeling at times, but we all strive to be happy. For some, it may be their goal for the day because it is not easy for them to feel joy. Those who are going through a rough time, like a breakup, think that they could only regain their happiness by getting that person that they depended on back. This isn't true. Happiness can be found in so many different people and activities that surround you throughout the day. You just have to find it in the little things and appreciate what is being given to you.
Someone once said to look at everything as though they are small miracles. With this mindset I could go through my day and list at least ten:
1. Waking up and being able to live another day.
2. Have running water to keep a good hygiene.
3. New clothes to wear.
4. Being able to eat three meals a day.
6. A loving family to argue with.
7. Car to travel.
8. Friends to cherish memories with.
9. Nature to entertain myself.
10. Shelter to sleep under.
These things are general and could be used almost every day as trying to look for the small miracles, so try to look even closer at your own life. Make the decision to be happy tomorrow. Do things that will get your spirits up and make you say at the end that today was a good day.