You’re flying the plane and each of your commitments gets a seat.

When you’re oversold (you’ve said “yes” to too many things) it’s not likely that your extra passengers will be standing in the aisles.

It’s more likely that they’ll be vying for seats that are already occupied.

Meaning: everyone on board will suffer.

There are many worthy causes, but the plane you’re flying has a limited number of seats. When you’re booked solid, saying “no” to new requests is out of respect for the ticketed passengers.