Why does the UI thread get blocked when running something in a different thread?

Using multithreading: Why does the UI thread get blocked when running something in a different thread? on newest questions tagged multithreading – Stack Overflow

Suppose I have normal button.Click event.

If I try to access UI elements from inside of this event I could potentially got unwanted behavior – or even an exception (when debugging). The usual exception in such a scenario is: ...cannot access UI elements from thread different than they were created in.

Since this is another thread (different than the main) why my UI is blocked when I perform time consuming operations in an event?

source: http://stackoverflow.com/questions/709340/why-does-the-ui-thread-get-blocked-when-running-something-in-a-different-thread
Using multithreading: using-multithreading