My friend Erick, who is a bit of a word hound (I'm assuming that's someone who is like a rock hound, but for words) has been trotting aspiration pony around the track. Here's what he emailed me:
I already brought up the phrase on a date last night. She noted that it's similar to "living vicariously through someone." But I think the newness of the phrase is in its recognition that being the person someone lives vicariously through can be a burden. But a burden mixed with being put on a weird pedestal, which is why the term isn't 'aspiration mule.'
Very true! And also, someone can make you an aspiration pony even if it's not something you want for yourself. For example, "Ken" never wanted his old college buddy to get married so the guy could "keep living the dream for the rest of us". Being single may or may not be what the college buddy wants for his life, but it's the aspiration that Ken projected onto him.