it’s not yours, yes - but it is you - or, it will have been you.
Organs aren’t simple possessions to be utilized, they are what constitute you. Sure, you can look at the utility function of a bit of human tissue shaped into an organ and decide that it may better serve someone else at a point where you are no longer - but that decision should be your own conscious decision, in my humble opinion.

I’m a donor myself, but I find it questionable to try and guilt or otherwise coerce or even worse, rationalize people into donating.