And also insisting others to play Undertale or any game for that matter and thinking lowly of them if they don't is the most obnoxious thing ever, cut it out
I completely agree; insisting upon the idea that it's the best game and anyone who thinks otherwise is wrong is unhealthy.
Granted I can see that a lot if time and effort went into making the game, so I do think the appreciation for it is somewhat justified. But having to see it as a crazed obsession pretty much everywhere on the Internet makes it seem overhyped.
Insisting people play it is one thing, insisting people love it is another. I keep trying to get my friend Noah to play it because I KNOW he'd love it, and even if he doesn't, at least he tried it. But he refuses. He'll break eventually.
I think it's more similar than you think it is especially given the situation. I don't think you want to have him play UT and then hear "wow, this is garbage"
Nothing wrong with wanting your friends like the games you like, of course, but please don't push it like a door-to-door salesman