Why is it so sad that some friendships don’t last anymore?

I’m just very blessed to have my two best friends that hurt my back since day one. Even though we are up and down from a fight but I don’t know what I’d do without them, even though they annoy the crap outta me like I am to them. I have a couple of friends I have since freshman year. The group of girls that I used to hang out with I had to let them go but it took me a long time to realize they were using me and some other friends I have from childhood we have to let them go too. My grandma would sometimes say, “You don’t have any friends! The only friend YOU have is god. You don’t need 100 friends to be cool. I only have three friends I trust.”
She clearly knows my best friends and I don’t know why she would say such a harsh thing.
It’s so sad that some people have childhood friends but they’re no longer friends and they just fight.