I once lost both my maternal grandfather and my paternal great-grandmother when I'm about 7.

But I didn't suffer as much trauma as Mari Okazaki in the game. But my mom once said she once dreamt of my grandfather once, perhaps she suffer some form of Mari-type trauma.

Mari's grief of losing her loved ones might be a reference to Asgore from Undertale, or even considered an exaggerated version of himself, even thinking of the traumatic events in her dreams, while Asgore didn't have such things. Considering Asgore and Mari both wearing capes and their relationships with a fighter girl (Undyne and Reira, respectively), Mari can be considered to be heavily based on Asgore plus my interpretations of people who have lost their loved ones due to traumatic incidents.

Actually, losing loved ones or the theme of someone dying that influences much of the game's events are not new to video games. If you know Clannad, then you will know what I mean. The protagonist of Clannad (who also appears to share Mari's surname Okazaki) actually have lost his relatives at certain points in the game (but actually he finally bring his wife back to life at the end of the game's extra story; unlike Mari who never bring her deceased loved ones back to life again). To the Moon, one of the most iconic games made using RPG Maker (which is what I use to build my game), have the story revolving around fulfilling a dying man's wish. Its "sequel", Finding Paradise, also follows somewhat the same motif. And coincidentially, Freebird Games (the company that made To the Moon and Finding Paradise)' founder Kan Gao actually said that the inspiration for To the Moon is his grandfather's life-threatening condition, which to me, somewhat resembles my situation when I have lost both my grandfather and great-grandmother in the same year.

Anyway, if you have any questions regarding the obsession with the theme of death and losing one's loved ones that comes from my personal experience, please leave a comment below. I will appreciate (and respect) your questions, and answer some of them.