After the interrogates Pinkie did to Spike, her yelling made him completely scarred for life that he refuse to be part of Pinkie Pie's party. Pinkie was confused but when she remembers what she did to Spike, she feels regretful and tries to cheer Spike up.
Thanks to MoJoK20X for editing this.
https://www.fimfiction.net/user/221451/MoJoK20X