Q: Why can’t fishermen be generous?
A: Because their business makes them sell fish!

Explanation: This is a bad pun (are there good puns?)!  Their business makes them ‘sell fish’ or shall I say “selfish.”  Yes, sell fish sounds just like selfish but there is a world of difference in the meaning.  A selfish person is someone who wants everything for him or herself.

Here is a video of some folks selling fish at the Pike Place Fish Market in Seattle, WA: