I'm trying to wrap my head around why some players don't think "I just don't like X" isn't a valid reason for a GM not to allow something. Especially when the GM gives zir reasons.
For example, I don't like metals that start with "adamant-" or are pronounced "mithril" (regardless of actual spelling) because I think they are the most overused fantasy materials.
What is this "zir" you speak of both here and in an XP comment above? Bit of Italian?-I see you're from Milan.
In any case, I dont like "Because I just dont like". Anytime I do something or houserule something, or change something-I always try to have reasons beyond that. I mean certainly a ruler (and you are Dungeon Master of The Table) could make an edict or any rule or change because he feels like it, but people feel much better if they give a reason based on some
logic or reasoning beyond mere sentiment and emotion.
I mean, what if there were no fantasy PC races in your game? You could say, "I just dont like elves, dwarves, haflings, gnomes, or orcs-I dont want any." and most people would ask 'why'? And might indeed be quite disappointed if they like any of those. Of course if you explain the history of your setting and the...
"Just So" stories of your setting, they may accept this. OF COURSE they could just leave and not start the game-
HOWEVER, players may become upset if some "I just dont like" comes up in the
middle of campaign when they already invested much time into their character and into the game only to have some oddball "I just dont like" come out of nowhere unexpectedly.
If you have any such sentiments which might come up, you should get them out of the way as soon as possible.
A GM can disallow anything they wish but they run the risk of losing players if they do it often without what the players believe to be good reason(s).
Also mind you though-the instant the DM gives a
reason the PCs can now
argue with them. (Albeit many people would think it rude to argue in the middle of play with limited time as opposed to doing it after the game) With "I just dont like" you as a PC can accept that or not, but you cant argue that it isnt
true. Who are you to tell someone what they dont like? Also mind you a player could just as easily say "I just dont like it" and NOT want to play the game the DMs way just as easily as the DM could say "I just dont like" and NOT run a game the way the player thinks it should.
It depends on how big a point of contention it is, whether the player leaves. I have never been in a game where I liked
everything. But in most of them I have liked more things than disliked. If thats the thing thats the tipping point for the PC they may leave. How significant keeping them is depends on availibilty of players. If you have a large FLGS and a lot of games, they may leave more easily, and you can just as easily replace them.
If your group is the only game in town, well thats different. But the DM wants to have fun too and they are putting in time they hypothetically dont have to -BUT so are the players. It comes up often DMs expecting players to make some compromises, accepting some aspects that they dont favor -but so should the DM
sometimes. If a player was an employee paid to work somewhere they might put up with it for the money. If a DM was paid to do with it, they'd sell whatever the people wanted to buy and "the customer" would "always be right".
As for why people dont always "just leave" if they dont like it.
1. It could be the only game in town, and having that game and what they do like about it outweighs other issues.
2. They may be in it for OTHER people. They may have a friend in the game who in contrast still likes the game. If they leave the group may be impacted and they dont want to be seen as ruining a friends fun. I have known people in such situations.
3. Relating to #2; if the DM or any of the people of that group should run another/different game they may remember how the player left the LAST time and for what reasons. They may see this as making them flakey, leaving over something that they didnt have a problem with and may not invite/allow the player in their game. Leaving game depending on the reasons could have consequences for other potential games in the future. (of course, a player that wants to get out of a game could simply
lie about the why. Albeit that in itself may be hard to them.)
EDIT: Random side note-I think you'd sound like an arse if you deliberately used the threat of leaving a game to try to leverage the DM into changing one or any number of things. (Although a DM could very easily
not run a game a particular way and say "well I'm not gonna run it that way" and somehow I think many people would see it the same way.)