Please understand, ladies. Starlight had a really messed-up childhood and Sunburst was her only reprieve from it. She needs closure for approximately 18 years of being consumed by her own anger and sadness, and Sunburst is the only pony that can give it to her.
Sunburst can also give her… other things, one of which she really needs. ( ͡° ͜ʖ ͡°)