What makes undertale so good? [Discussion]

So about 3 days ago I started playing this game because I've heard from so many people that it is so good.

... So I decided to give it a go.

My first run was okay (neutral) I didn't particularly think it was an amazing game then my friend told me to try playing it again this time without killing anything.

So my second run again... good game i guess, but nothing to be 'amazed' about. Then my friend told me to play genocide, he told me it is the best story out of all 3. And I am just thinking, what about this game made people go "wow this is awesome I'm going to play again this time killing everything I'm sure that will have better ending".

So I played genocide and I just finished and yet again... it was nothing special.

I'm not trying to be a hater I just want to know what I am missing out on, everyone says it is such a great game what am I doing wrong?

I think one of the reasons is I played through it too fast, and for some of the more difficult bosses I got frustrated which made me hate it a little, but still after genocide I was expecting some amazing ending that would make me fall in love with the game but I didn't get that...