This.
And to your empathy thing, imagine if everyone were as selfish as you in real life. Your teammates don't have to do anything to "win" your courtesy, your courtesy should be automatically given to them, just like in real life.

EDIT:
After reading through your posts, I realized you're a lot more selfish than I thought.
Your attitude is disgusting too.
If you start to realize that nobody likes you in real life and you have no friends I wouldn't be surprised.

But let me try to convince you from your point of view:
You will enjoy the game a lot more if you cooperate with your team. You will enjoy the game more if you have a positive attitude than if you have an amazing KDA, I guarantee it. If you have an amazing KDA but you forced the best top laner in NA to play support, that support will feed, you will get mad, you will lose LP, and you will have to deal with a 10-0 Vayne.

You will have much more fun in the game where you get a good KDA and the rest of your team does too, allowing you to push through objectives and win the game.

Besides, what good is a KDA when you constantly lose? You will drop lower and lower in elo hell and then you'll realize that the game was a lot more enjoyable in the upper elos than having to play with idiots and watching them do the stupidest things.

because it's probably their best role and if you take it, you're consigning them to a role they're worse at

Why would you even start this thread if this was going to be your answer? It's a pointless thread because every good reason can be refuted with this. I am ok with this if it's normals, but don't play ranked.

Because u are a good guy?

Because chances are they're better than you.

I've also been in that situation before, we ended up with 2 adc's and lost horribly.

Also, and this is probably the best piece of advice, try to learn every role on the team. That way you can enjoy playing the game no matter what you get stuck with.

Just to mention, because they ask for it, doesn't mean it's their best role. Just that they want to play it.

Because that's currently how the system works - whomever calls the role first, gets it. It's the only real method we have of determining who gets to play what and it keeps most everyone happy. If you don't get the role you want, it sucks, but you can always play something else. Help out the TEAM, not just yourself.

Just realised how nonsensical that is. Beyond 15 min, there is no difference between jungle/top bruiser or support/mage/some jungler/tops.
Op needs to understand the game.

okay
fellow calls
"Top"
unfortunately he is last pick and everything but Jungle is taken by the end
he stated after calling Top
"I fail at Jungle, hard"
game starts
Jungle Yi
Mid and Top come and help with Blue
Yi gets it
Yi then dies to wolves
Yi comes back
tries again
dies again
over and over and over and over
game ends at 30 min surrender
Yi is /5/3
every death is an execute in the first 10 minutes
this is why you let ppls take the lane they called or at least ask what they CAN do

Because it's likely to be there best role, and you want people to play there best.