It's not hard to see why people find the concept of FIRE — short for financial independence, retire early — alluring.

Adherents to the movement aim to stash away as large a portion of their income as possible in investment accounts. The more aggressively they save, the sooner they can reach their so-called FIRE number, the amount of money in their account which they can withdraw from in perpetuity to replace income from a 9-to-5 job.