I'm doing a master in social sciences. Moodpanda is nice (at the time being, using it only since a week) for tracking your mood because (I use the website btw, not the app)my mood fluctuates. When I feel bad I can't remember that I felt good the day before. Not remembering makes me think negatively about everyting because I tend to think life was always bad (or that the good won't come back). Moodpanda helps me remember. I can look at the past two weeks and see that my mood was pretty good. So when I feel bad again, I know that it's only temporary and I will feel better again. And that feeling bad makes me see things negative and I shouldn't take those thoughts too seriously.
Also, (depending on your privacy settings) you can read other people's mood and others can respond to your mood (by giving hugs primarily). This emphasizes that you're not alone in feeling bad. Also when happy, I like to give people kindness, support and advice. It's empathy.