Airlines will oversell flights, and sometimes there are issues finding seat assignments for larger groups or families travelling together. If you're travelling alone and have flexible plans, you could opt to take the next flight for a potential upgrade (or travel voucher, which you could use on a future flight to purchase an upgrade).
"In situations when flights are oversold and there are seat assignment issues with couples or families needing to sit together, I highly recommend you offer up your seat to help out other passengers," Spagnola explained. "Flight attendants appreciate the help trying to keep all of their passengers happy, so if you're travelling alone or don't mind being separated from your group, be flexible. I've done this myself many times, and have been put into business or first class when there was availability."