ELI5:why are dentists their own separate "thing" and not like any other specialty doctor?

Let me take a moment to explain why you're being downvoted. One of the rules of the internet is "There are no girls on the Internet." This does not mean what you probably think it means. It doesn't mean "every single human using the internet is male." It'd be more accurate to say "gender means nothing online."

In real life, people like you for being a girl. Put bluntly, they want to fuck you, so they pay extra attention to you and pretend what you have to say is interesting, smart, or clever. On the internet, there is effectively no chance to fuck you, except for possibly in a very location-specific culture, like the subreddits dedicated to individual cities.

This means that the advantage of being a "girl" does not exist. You don't get a bonus to conversation just because the guy you're talking to wants to put his cock in you. When you make a reference online to you being a girl, you are begging for attention. You are trying to get your girl-advantage back, whether because you feel naked without that advantage you've had your whole life, because you're too vapid or uninteresting* to hold an interesting conversation without it, or some other reason I don't feel like explaining right now.

* (Note: I'm not trying to say you are vapid or uninteresting, only that it is one possible reason why some girls claim their gender online.)

Now, taking this into consideration, you might ask why everyone is assumed to be men, if gender doesn't matter? Well, think about the internet in its infancy, when it first started getting popular in the mid-90s, and think about the way men were treated compared to women then. I'm not saying it's right, only that it's true. Since gender doesn't matter, if you wanted to be accurate, there would be no gender-specific pronouns like "he, him, his, guys, men, dude," etc online. But it's a lot easier to simply pretend everyone is a guy and use the preset set of words for them than it is to use gender-neutral words like "its" or "their," or to make new gender-neutral pronouns and make them widely known/used.

The one exception to this rule, the one way you can get your "girlness" back on the internet, is to post your tits. This is, and should be, degrading for you - you are admitting that the only interesting thing about you is your naked body, and you are mentally incapable of holding my attention without it.

TL:DR - TITS OR GTFO

/r/explainlikeimfive Thread