I get this from time to time. Someone insists that I can't be "that happy" all the time. It is rarely said to me directly, but I hear about it.
Well, for the doubters, you are correct. I am not perfectly happy and joyful all the time. Things happen that aren't in my perfect plan, things happen that may seem bad on the surface. Here is the secret, though, to my joyous demeanor: when those things happen, I don't dwell on them. If I do think about them, I search for what is right with the situation. This leads me to be happy and joyous most of the time. I try to refrain from complaining because what I complain about, I am giving my attention to. What I give my attention to, I bring into my life. If I want to continue to be happy, I give my attention to happy thoughts.
Now, I understand that many think this is just a bunch of woo woo new agey hoo haw. I get that. I used to think that, too. But there is science behind it. You can change the story that you tell yourself and in doing so, you change the way your brain's synapses fire. Changing the way your synapses fire change your brain. Changing your brain changes your being.
Our bodies are amazing creations, and the brain is quite possibly the single most interesting thing that I have ever endeavored to study.
Don't take my word for it, check out the video of Dr. Joe Dispenza's talk at TedxTacoma. It is amazing and gives you a scientific explanation of the reason that I can be happy and joyous most of the time.
It IS a practice, though. You learn the information, you put it into practice by acting on the information that you have learned, and eventually it is just part of your being. It is who you are. You have changed your brain!