Why can't I left click on my Macbook Pro?
Let's just say it's a story of unknown Bluetooth interference or a paired Bluetooth device that’s in range of the Mac. 

I had this very issue last week on my Early 2013 15' MacBook Pro with Retina Display. Normal (Left) mouse-click wouldn't work under any conditions while running OS X, the built-in Trackpad, Magic Mouse or USB Mouse, while the secondary-click was fine on all.

I booted my MBP with a Linux Mint Live USB Flash Drive and the built-in Trackpad, Magic Mouse and USB Mouse all worked, both normal-click and secondary-click.  So at least I knew the built-in Trackpad, Magic Mouse and USB Mouse hardware was fine and it was looking like a software issue.

As another trouble shooting process I made an OS X USB Flash Drive Installer, figuring it would boot clean and the built-in Trackpad or Magic Mouse would work fine since it worked under Linux, however during the install... no normal-click would work, while the secondary-click was fine.

Turns out my Magic Trackpad, which I was not using and was in the closet, was depressed by a book that fell over on to it.  Because it was connected, even though not in use, it was causing the other clickable devices to fail on the normal-click while the secondary-click still worked.

This drove me crazy for a couple of hours because the book fell when I was not in the room, so I never heard it or suspected the Magic Trackpad to be an issue.  Linux Mint Live USB Flash Drive worked because the Magic Trackpad was not paired to it and therefore couldn't and didn't interfere.  Unlike in OS X it automatically connected.

What a pain it was but who doesn't love a good mystery!

Do you by chance have a Magic Trackpad lying around? 

——

Or a Bluetooth mouse in a travel bag - if that mouse is pressed down, you can’t ever get a left mouse up event from any other device.