It's because we're too bitter from not eating donuts - we have to share our misery with everyone else to compensate.
See the earlier picture of a vegan. We're not tasty.
Doing it for health reasons and that parts working. Also doing it because my daughter's been a vegan for awhile now and does really well with it...figured I'd show some support for her. That doesn't stop me from missing donuts and bacon though. :|
Ironically, you did just as the joke predicted you would. So that suggests that vegans are vegan so they can tell other people about their dietary habits. Good joke.