All things change.
Eight month old babies can change from happy peaceful and drooling, to screaming comparable to fingernails scratching a chalkboard in annoyance and a police siren in volume. No one can imagine the full effect of such a sound until it's dropped in bed with you at 3:30 in the morning.
Jobs change. The practice of one job as a career from graduation through retirement is long forgotten in this age of short attention spans and our quest for bigger better and more.
Landscapes change. My home town is now as unrecognizable to me as Michael Jackson's face pre plastic surgery. Marysville WA was once a mixture of 1930's era homes, ma & pa shops, farmland, grassy fields, and forrest, it has since been paved over and turned into modern day suburbia complete with a WalMart and a Cineplex.
Circumstances change. Traditional marriage vows include words like richer, poorer, sickness, health, better and worse. These words set up a realistic expectation, (possibly a warning) of what's to come. Most people expect to experience those highs and lows some time with in their lifetime as a married couple, but no one ever tells you all of it will happen within the first couple years.
What's my point?
I love job titles where your job description is "Whatever we come up with for you to do" or "Stuff that no one else has time for." There's nothing as enjoyable as preparing for a 3:30-midnight class and finding out you schedule has changed to start at 7:00am the first day of class. Even better is when you discover that change around noon that day. Five hours into class, you get to walk in and introduce yourself - "Hi, I'm your teacher and we'll be here for the next six weeks. Please show up on time. Any questions?"