sans's 20th birthday was a few days after papyrus left
the day after that he was going to college and would be separated from all of his friends, and papyrus' disappearance really hit him hard
so grillby gathered undyne, alphys and muffet to throw a bomb party for him
and he was so happy he actually cried

he kept in touch with them throughout half the year he attended college before they all got really busy with work and school
sans proceeded to move back home and sign up for online courses to be closer to his friends and to be home when papyrus came back