The title says it all really.
Tabletop Games are just games that are played on a tabletop. Games are an activity providing entertainment or amusement and as such should be fun. So how does this turn into a principle rather than just happening by default? Well... Not all games are for everyone. Just as not all foods are for everyone or all cars are for everyone, fun whilst playing games is a matter of personal taste.
Across the spectrum of Tabletop Games you have a lot of different formats and within those formats you have a lot of different styles of play and indeed themes for the games in question. Given the sheer variety of choice on offer you will find that not everyone will want to play game X or indeed what to play game X the same way as everyone else. In Robin's Laws Of Good Game Mastering there are a number of Player Types defined which help with to understand this but without reposting all of that I'm going to give examples from my own experience.
e.g. Magic The Gathering (and I suspect the majority if not all CCGs) has a variety of play styles and also has a variety of players. Some play styles will be "aggro" based which really just means beating your opponent quickly and efficiently. Some players will also only play that style. Now in a competitive environment you might think that having fun isn't really part of the deal and that's where I'd say you're wrong. Yes I know the game is competitive but it's still a game and therefore should be about having fun. If you're finding that your only reason for playing Magic is to win rather than playing it to have fun but also trying to win then I think you're missing the point. The same goes for other competitive games and I'd suggest moreso for cooperative games.
Why moreso for cooperative? Well if there are 5 of you playing a cooperative game and only 3 of you are having fun then the game isn't a success. Your individual play experience might be but the overall game isn't. This can lead to people not wanting to play game X or Y or whatever simply because they see it as "Not Fun".
This principle also dips into things like "giving everyone their time on camera" which is really just about ensuring that everyone has a chance to play and isn't just sitting there with the game going on around them.
Ah but you also said "Including you?" in the title.
Yeah, I know. I've played many games many times over the years and my focus is usually on ensuring that the other players (particularly RPGs) are having fun. During the start of this 2nd Chapter I was reminded that I also have to have fun otherwise it kinda defeats the whole point of this being my hobby.
What do you do if the other players are having fun but you aren't? Try something else.
Whether that be a different game altogether or if you're the GM in a RPG then try a different game style. e.g. if your players seem to enjoy "combat heavy" games over anything else but you find them uninteresting then you have a problem. This can really only be addressed through working together with your fellow players and through mixing up the styles of play even to the point of having a hybrid to give those who want "combat heavy" what they want and those who want something else get what they want including the GM.