>>7615231People don't say anything about things they don't like because of the Golden Rule: "If you don't have anything nice to say, don't say anything at all."
In their opinion, it's better to just privately dislike something. That way, people who like what you don't are able to enjoy it. If you genuinely have cause to speak out against it, feel free to because there might be substance to your argument.
For example if a bunch of people are getting tattoos and you don't like tattoos in general, keep it to yourself. If you know that the tattoo parlor they're all going to is unsanitary, speak out about it.