She never lets anyone chase her when she is vulnerable
When she feels the urge to cry, she holds back until she can escape into the bathroom to let her tears run. When she is upset, she invents an excuse to cancel her plans, and avoid having to publicly pretend. And when she has a depression, she does not send messages to her friends to talk about or make any posts on Facebook. She suffers in silence.
She does not see the point of sharing her misery. Her friends consider her funny. Bubbly. The person who solves the problems. They come to her with their problems. They expect her to know what to do. They expect her to manage.
So she nods and offers solace when they complain about their "almost perfect" life, even if she wants to scream out loud so that they know what the real problems are. She's struggling with something a million times worse.