Like when you know exactly what Setsuna is preparing to do here, basically just run off to her near inevitable death in order to save everyone else- this whole scene of them at dinner is a bit sad, with Setsuna making sure to go above and beyond to keep them happy, and make sure the last memories they have of her are pleasant ones.
That said, this isn’t something she’s necessarily unhappy about; her attitude throughout the scene is incredibly chipper and positive- part of it is that she doesn’t want Love to get any idea that anything is wrong, of course, but I think that part of it is that she is simply at peace with herself regarding what she is about to do. That she is making the right choice.
I think in many ways Setsuna also sees her life as not as valuable as the others anyway, for a few reasons. It is worth noting that she did already die once, and love, magic, friendship, etc. essentially gave her a second chance at it. When that happens, I can kind of understand making feeling more willing to sacrifice your own life for that of others, because, well, you already died once anyway.
Another part of it, though, is that Setsuna really clearly in many ways feels that her life has less worth. She’s been consumed by guilt for a long time, even before she started being Cure Passion, and on some level she does want to be free of that. Even after she became Setsuna and earned their trust, she always had the belief that there was no way that they could possibly trust her after everything done, somewhere in the back of her mind.
And she simply, to a degree, sees herself as less important than the others because of that, because unlike them, who haven’t done anything wrong, she has a lot of crosses to bear and if she can make things better by sacrificing herself? And if in the long run that will (in her mind) make her friends happier? She’d do it in a heartbeat.
Like Setsuna’s character kind of has this constant undercurrent of self-loathing throughout the series that I think probably will take a long time (and therapy) for her to really unpack and be able to deal with.