[noob question] someone picks adc, I pick bot ashe... adv gets very angry and nearly rage quits...

Can someone explain to me why I a player who picked adc raged really hard when I picked bottom lane Ashe? (sorry for being a noob but I've only been playing for less than a month)
is there some kind of rule where only one member can carry or something? I don't know how to carry a game because I'm not very good. I main Ashe because she has a beginner skill cap but very good map presence (plus, I liked her backstory in the comics). As far as I know, there are five players:
one jungle,
one top lane, usually melee
one mid (I assume top, mid and jungle gets to soak up more exp during early game)
then, there's the bottom + support duo. I usually pick bottom since I prefer ranged and I welcome anyone who shares the lane with me be it support or not.
so, this one guy (with the attitude of an infant) threatened to AFK after I locked in to ashe because no one called dibs on bot lane. he carried the game in the end which was what he intended to do. I played the game by supporting the team which was what I intended to do. So is there something about LOL roles etiquette that a nood like me should be aware of? (especially during MSI a lot of players are in a very competitive mood while I just want to have fun)