Someone please explain.....

why I should give a takeback when my opponent makes a stupid move that results in his losing his queen?

You don't have too, but you can if you want.

In a word "Karma".

@MrHandy1 #1
In my personal opinion take-backs in chess are artificial and can bring extra "guilt" and "anger" elements to the game of chess (For example : "I did let you take back that queen blunder but now that I blunder you want to play on ??"). And where is the end ? How many take-backs per game do you allow ?

Take-backs can imho only make some sense if you are playing without a chess clock, in a very friendly chess game where nothing at all is at stake.

In otb chess there are no take-backs. In on-line chess I prefer to have take-backs off in the settings.
That results in none of these heated take-back discussions during or after a game. Instead there resigning or rage quit after a blunder or mouse-slip, and the start of a fresh new game.

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"There are no take-backs—just as in life. You must think before you move." --- Susan Polgar

In lichess I have take-backs off in the settings.
A few months ago I noticed a strange increase in requests for take-backs. I always refused (if I'm wrong, I admit my mistake): many players were angry and insulted me.
As soon as I realize that I'm about to lose: 3-4 take-backs...
I lost my queen: take-backs...
In my opinion, they have to go to play with dolls.

Right you shouldn't accept takebacks in general. I don't put the off in the settings because there is one exception where it is polite to allow a takeback. If your opponent has an OBVIOUS mouseslip. In these cases I always allow a tackback so the opponent can play the move he wanted to even if it means I lose the game.

I think you should never "ask" for a takeback. If your opponent makes a mouseslip, you may offer one, but I don't like it the other way round.
Just like with draw offers - imo, it's always the better side who should offer the draw (unless they are clearly just shuffling their pieces without any plan - in that case it's maybe ok for the worse side to ask for a draw).

In my preferences I chose takebacks - Never. It's against the rules of chess.

I thank lichess for having this preference available.

By the way there is excuse for mouse slips. We can click on the square we are coming from then click on the square we are going to, which makes a mouse slip impossible.

I meant "By the way there is NO excuse for mouse slips."

Drag and drop can be laggy and even without using drag and drop it can happen but of course that never happens through an oversight...

But for some people it's hard to understand that other people can do these things from time to time mistakenly.