Don't Amanda and Jim have more in common (in their personality traits, I mean) than anybody else?
Amanda Wingfield's personality tends toward the eccentric. She lives in a fantasy world dominated by a past that could very well be fabricated to suit her vision of herself. She puts pressure on her handicapped, shy daughter to become something in life, if nothing else at least someone's wife. She insists that Laura have some ambition, she feels the same about her son, Tom, both suffer from a lack of interest or enthusiasm in life.
Jim, the gentleman caller and Amanda share the same sensibility when it comes to how to look at life. Jim encourages Tom, he is an optimist, always looking on the bright side of things.
He also brings the light of joy into Laura's sheltered life, even if it is only for one night. Amanda and Jim share an understanding of reality better than any of the other characters. But Amanda's reality is unstable, where Jim's is much more realistic he is anchored in a reality that includes a fiance, improving his position at work and a cheery attitude about life. Amanda shifts from present to past too often to be considered stable.
Amanda at her worst, is dominating, critical and obsessed with controlling her adult childrens' lives. Jim comes off as more of a free spirit, just enjoying the company of Tom and his family for a night while his fiance, Betty is away.