I keep having this one agonizing thought.
Unless you were really into British pop music-based television, the introduction to One Direction for almost everybody in the world was "What Makes You Beautiful" - and those opening lines, no matter how memed or overplayed they became during that nostalgic autumn of 2011 - are Liam's vocals.
Whether you ended up being a stan with posters all over your walls who cried in your middle school bathroom when Zayn left or you were someone that got swept up in pop culture hatred and called them annoying or overrated - that song was likely the first impression One Direction ever left on you.
That means Liam is the first thing the world ever heard of One Direction as a group, so for him to be the first thing to leave, truly permanently leave the world behind, makes the gaping hole feel that much larger.