Why do people throw away stuff in their trashcan?

Exactly!!! finally someone got it!!!

Well you should have said that it was illogical to throw away stuff in a trashcan if you live on Sesame Street.

Answer to OP's question:

It is actually a very profound and intriguing question. How has human society invented the concept of "trash," when it does not exist at all among other animal species? And how has the entire human race (or most of it at least) come to the mutual consensus that trash should be thrown into trash cans and disposed of? Is it that humans are instinctively repulsed by the notion of living side by side with things that they consider to be worthless (aka trash), or is it merely a habit that has been passed on through generations? These are all intriguing considerations raised by the OP's original question.