Teenagers desperately want to fit in with their peers, but years later, as adults, they want to be different. They don't want to be just like everyone else. (For proof, tell a chef that her duck á l'orange is average and you'll be banished from her restaurant.) Being average can be an insult at times and something to aspire to at others.
Workers' attitudes evolve in the same way when discussing income. At the start of your career, especially if you're beginning in today's roller-coaster economy, you're thrilled to earn the average American salary. Once you're an industry veteran who's put in decades of work, you probably hope your salary has increased over time. Not because you're greedy but because you'd like to think your years of hard work are being rewarded.