Let's start with the assumption that every time we write something, we're trying to evoke a mood in the reader. One valuable tool to use here is description of the setting. The words you choose when you're writing about the environment your characters are in will shape the way the reader sees your characters and their experiences.
As a writer, you want to be able to convey as many different moods as possible. I've found I'm really good at articulating some emotions and really, really terrible at others. Here's an exercise I use to help me build my descriptive skills:
Every couple of days (and this is important - you could do this exercise every day if you wanted to, but don't go more than a week without!) stop what you're doing, and really look at the spot you find yourself in at that moment.
You don't want to think about ANYTHING besides really looking at whatever is around you. What do you see? What does it look like, sound like, smell like? You don't have to write a word of this down, you know: the point here is the observing. Spend a minute or two focused on being very aware of your environment.
Now stop. Shift into writer mode now. How would you describe this scene if you were happy? How would you describe it if you were sad? How does the landscape look to an angry person? How would it be seen by a person bubbling over with excitement? Again, you don't have to write this down - just think through the language you would use to express that mood to the reader. I usually do this for a mood or two and then move on with my day; over the course of regular practice, you'll have an opportunity to practice viewing the environment through a wide range of emotions if you switch it up each time.
That's it. That's the whole exercise. It'll make you a better writer. When you're writing a scene, stop and ask yourself how your characters see where they are. Share that information with your readers. It'll make your text better and more rewarding.
Here's the nifty off-label application. Don't forget that you are the main character in your own life. Every now and then, stop. Take a look around. Become aware of how you feel right now. Ask yourself "How would I describe this if I was happier?" Just going through that process boosts the mood. It's a little jolt of positivity that can come in really handy sometimes.