Assign Workflow, plugin message in MS CRM 2011

One observation regarding to the Assign for workflow or plugin.

I want to do something on Assign of a record, may be workflow or plugin is d option.
Workflow
If its workflow, it fires while assigning, but not if assigning to myself.
Any reason why?
Plugin
This fires each time we assign a record even if its myself or other users.

Thanks,
yes.sudhanshu