because you don't want to sacrifie one of your relatives is stupid and selfish

A lot of people says this maybe because this is just a fiction and they want to say things that make themselves feel good, like calling out

But try to empathise with Eren, imagine yourself at the lever of the trolley problem, on one track is a thousand people, mostly strangers, some acquaintances but generally people whom you're not close with. Then imagine there's only one person on the other track, and that is your spouse/mother/father/someone very close to you. Where would divert the trolley to?

You can try to rationalise like Eren did about how saving the 1000 people would for the greater good, each and everyone of them have someone they love/loved just as much as you love the person on the other track, how that one person's life does not outweigh the lives of 1000.

But I'm willing to bet, if push comes to shove, most people, like Eren, aren't willing to accept an end like this.

Because humans are naturally selfish.