Star comes from a life of being (arguably rightfully) considered reckless and irresponsible by everyone. As far as we know Pony Head was her only close, same aged friend, and she's worse than Star. Then comes Marco, a sensible young boy who, while being her best friend and peer, also believes in her, doesn't blame her too much for her mistakes and admits when he has some fault to share. This makes the matter of "trust" an extremely important part of their friendship, and every time it gets "betrayed", in more or less serious ways (Blood Moon Ball, Star on Wheels) Star suffers.
In Banagic Marco outright tells her that he didn't believe she could acquire the wand all alone, Star feels hurt, reacts in a childish (and, I admit, awkwardly executed) but explainable way.