The faffer takes forever to get out and go in the morning. The whinger won't stop complaining. And the waverer can't make a decision, come hell or high water. (No, I didn't know what these words meant either...)
Then there's the fussy eater, the drunk, the person who needs to be pampered, the person who is stressed by everything. I'm sure you know other types of people whose habits could lessen the joy of your trip, if not ruin your travels altogether.
These and the other aggravating travel companions will detract from a trip. They may even cause you to miss things you specifically went to see. What good is that, after you've spent your hard-earned money to have an adventure?
Other travel companion issues:
You may end up traveling with someone who wants to eat in the finest restaurants when you want to eat street food. Or someone who wants to buy all meals from supermarkets when you want to taste the local cuisine at highly recommended spots.