Why does my GMOD file have to validate every time I start up Steam?

Seriously. Whenever I start up Steam, it validates my GMOD file. It’s really annoying. Is there anything I can do?
It also does this with my 13 beta.

Mine has the same problem, but it’s all my source games.
I usually just browse the web while I wait for it to finish.
I’ve looked it up and people suggest you de-fragment your games.

This problem happened to all my source games too, I waited so long that I eventually had to go to school, came back and it wasn’t done validating.
After awhile I didn’t play and steam had to update, came back and it was fine.

For me it seems to happen when I sign into steam on a different computer. If you have used another computer for steam sign out and unistall steam ( un-installation isn’t necessary )

what

you have to unistall steam, but you don’t have to un-install it