What a lovely thread. 
for OP.
I'm generally rather happy, in fact one of the people on a course I went on recently said I was the most positive person she had ever met :) I know I'm incredibly lucky as two of my immediate family suffer serious depression.
I think a lot of it comes down to what SirRaymondClench said about being grateful for things. I feel incredibly lucky for so many reasons. Every day I find lots of things to be grateful for. Every night before I go to sleep I list five things I am grateful for that day and that means I go to sleep in a happy frame of mind.
Like Unicorns I've been doing mindfulness meditation with Headspace for quite a while, I recently subscribed to the full service so I am working my way through the different series. I find it so refreshing. I try and do it in the morning so it sets me up for the day. Since I started I've found I'm more present in every moment which I think really helps with happiness. I'd definitely recommend it :)
Another thing I do is that first thing in the morning I get out of bed, open the curtains to see the day and have a bit of a stretch.
Really though the main reason I'm so happy is that I find joy and beauty in so many things. I've had a less good few months as was finishing up six months of chemotherapy and have been fairly ill with it so unable to go out much. Even on the grimmest days when I was waking up to go to the bathroom up to ten times a night, I still managed to find a glimmer of joy in seeing the beautiful night sky through the landing window. The sky is the most beautiful purpley grey colour around dawn :)
Nature is definitely a big source of joy, as is music. I also love learning so I read lots of non-fiction books about different subjects, I do lots of puzzle books, cook, potter in the garden. Lots of cheap/free things. The feel of sliding under the duvet on a chilly evening, walking barefoot in the garden, yummy food, playing board games with my family and friends...
As whenever I walk past a mirror, I smile at myself, or if I'm feeling silly I will make a funny face :) that always makes me happy. I'm that crazy girl walking round town smiling at everyone, it's usually only the older people who smile back! I try to embrace my inner silliness and do silly little things like jumping off the bottom stair, or walking in muddy puddles in my wellies.
I've rambled on loads and probably sound a bit weird 