Laura suffers from what we would today call social anxiety, but in Tennessee Williams' time might be referred to as being "painfully shy." Her ability to function in the everyday world of work and social interaction is severely limited by this psychological issue. We can certainly take a guess at what may have made this condition develop in Laura. Her self-consciousness over her disability has contributed to it; the leg brace she is forced to wear is very noticeable.
Laura's mother is also partly responsible. Amanda is very outgoing, aggressive and flirtatious, and it is very difficult for Laura to assume any form of social dominance because she is not capable of matching her mother's extroverted qualities. Amanda tried to get Laura to fit in by sending her to business school, but Laura's shyness and awkwardness prevented her from being able to participate.
It also seems that Amanda's own somewhat delusional personality (she has a tendency to live in the past) interferes with Laura's ability to become more socially outgoing. Amanda insists Laura remain "fresh and pretty" for any young men who might come to visit. But in the scene where Tom brings a friend home for dinner (the "Gentleman Caller," who is also a former schoolmate of Laura), Amanda dresses in an inappropriate youthful outfit and flirts with the young man herself. This causes Laura some embarrassment, party because she is incapable of behaving in this way. Laura is so sheltered by her home life and what she finds "normal" that she misconstrues the Gentleman Caller's intentions when he tries to break her out of her shell by dancing with her and kissing her.
At the beginning of this scene, the description of Laura's appearance compares her to the glass animals in her collection, a hint that she is destined to be looked at and admired, but unsuitable for physical intimacy:
The arrangement of Laura’s hair is changed; it is softer and more becoming. A fragile unearthly prettiness has come out in Laura: she is like a piece of translucent glass touched by light, given a momentary radiance, not actual, not lasting.