When you grow up, your heart dies. Don’t let your heart die.
Sunburst left and Starlight Glimmer stayed behind. She tried to make new friends, but it never really seemed to work. She even ended up with false friends who betrayed her trust. Still thinking of Sunburst, she wasn’t able to keep up with other ponies her age.
The older Starlight got, the more bitter she became. Her friendships were always short-lived because she was used to it. She let her heart die.