I think an activity becomes sad when the person lets their desire to do that thing get in the way of them experiencing new things and achieving goals, or discovering what they actually want in life.
Of course, one man's sad life is another man's exciting and fulfilling life, it all depends on what you compare it too. One can play video games for 2 hours every day and say to themselves, "Well, at least I don't play video games for 4 hours every day." And the person that plays games for 4 hours every day can compare himself to someone who plays more, and so on.
The point is, every moment of your life is precious and there is always some way you could improve or enrich it. You only spend an hour each day playing video games? Fuck that. Try spending that hour doing something new, or playing a different game, or playing that game you usually play in a drastically different way, or with a different person, or in a different place/situation. Don't rely on reliving the same moment to achieve that euphoric feeling to be happy. That goes for everything.