Ever since Sonic Generations, it’s been tradition for Sonic to gather up all his friends and throw a party for them to show his appreciation and love for every single one of them. He’ll search high and low to find every single friend he possibly can and invite them to have fun. He’s even reached through dimensions to invite the likes of Silver, Blaze and Marine.
Even Eggman gets an invitation. Because although they’re often at odds with each other, Sonic can’t imagine his life being anywhere near as good as it is now without Eggman there.