It even lets you hum along happily for months at a time, thinking that you're perfectly acclimatized to a place after living there for years.
No, homesickness is a nastily patient little bugger. It waits until you're weak. It knows that the time it can smite you with greatest might is when you're going home at midnight from your hard-won non-expat friends' house, having consumed a repast consisting largely of several glasses of bubbly. When the craving for your favourite crap post-booze-session meal hits you hardest, and it is a thing that you cannot possibly hope to obtain from many miles in any direction: a refried-bean-and-cheese burrito from a mustachioed Mexican in a cockroach-infested taco truck whose English makes his bellow of your order to the cook sound like bin-and-chiss.