Turns out you are not the only one: https://core.trac.wordpress.org/ticket/32095
It looks like this was removed in a recent update (4.2) and I can see why they did it. User interface can be confusing at times.
You can read more about the removal here: http://wptavern.com/how-to-restore-the-link-title-attribute-removed-in-wordpress-4-2
There is a WordPress plugin, however, that seems to add it back for you. I have not tested, but it should work:
https://wordpress.org/plugins/restore-link-title-field/