If you're my manager and I really like you enough to actually want to help out it's because of 4 main things:

1. I'm compensated appropriately
2. I know my extra help will be appreciated and include extra compensation
3. This isnt a regular occurrence and the position isnt chronically short-staffed
4. You bust your ass to make sure things get done before you try to pass the work on to someone else.

Without those 4 things I don't care how cool of a person you are or how great your personality is, you're a shit manager and you're doing a shit job and I'm not going to pick up your slack.