Adding a Tooltip to tell users why a button is disabled is a good way to reduce frustration!
Disabling a button could be frustrating and confusing to users in many cases, as yes users usually cannot interact with a disabled button/object. Adding a Tooltip that guides users on the reason an object is disabled is one way of interaction that would reduce users' frustration.
Take this example: Add Course button is disabled till all mandatory fields are filled out. When the button is hovered, a Tooltip will display to explain why the button is still disabled.
-Note: When all mandatory fields are filled out, the Tooltip content could be changed to explain the button functionality, or simply do not display it if it is not needed anymore.
download bmml source – Wireframes created with Balsamiq Mockups
Update: I just found this answer to support the idea.