The main focus is a young male, Punpun Onodera, from around age 12 to his 20's. You watch Punpun as a little kid commit to foolish ideals and promises he makes, and when they go awry you watch him get depressed about it. His troubled life at home only exacerbates the cycle. As he grows older, these setbacks and more cause Punpun to shut people out, and make more unnecessary commitments to himself. Sounds like joy
You could probably say this is going to be depressing, and at times it will be, but it's more that the manga is about everyday existence. To spruce it up, GP is drawn well and written cleverly enough to keep your interest!
Take the picture above, Punpun is mucking around with his friends - and which one is he? The bird stick figure thing at the front. Why that design? Well, a part of Goodnight Punpun's charm is that the author, Inio Asano, likes to shake certain manga tendencies.
Most MC designs are rather plain, and these characters 'become incredible' at some point. Punpun is 'plain' as a character. So you could say this design is having a dig at design conventions. There are a few other reasons for the design that I could suggest, though you'll arrive to that conclusion as you experience the manga. My main point here is that the author goes out of his way on a few occasions to play around with or criticise manga writing. On that note, there is love too...
In the very first chapter, Punpun's crush leaves the school. Then Aiko arrives:
Sounds like what a lot of little boys do in a few stories? Or IRL
Compare this to the outcome, by say chapter 140. You'll be
rocked at that point. In fact, I recommend reading chapter 1 after you finish Goodnight Punpun for that hindsight.
Anyways, I'll add more at a later time
Edit - when I do, I'll touch on some of the darker/human elements to it, coz what I wrote so far may be misleading.