Because it's tiring. When you're tired, your tolerance levels go right down and so little things can sometimes become very stressful. Decisions (eg navigation decisions) can also become difficult when you are tired, so you could get lost more easily, for example.
If you don't know your riding partner very well, and they don't have the same outlook on touring as you do, all kinds of things can result in arguments.
e.g. Fitness levels will differ, so it could be easy for your touring buddy to get really pissed off with you if you keep dropping them. Equally, you coudl get pissed off waiting for them. THey could be a competitive type, and so you end up riding in this terrible atmosphere when they are trying to drop you, burn you off or compare themselves to you all the time.
In the morning, they could be really disorganized and take ages to get their panniers together, whereas you could be really efficient. After several days of this, it could get very irritating. They could have a completely different view of personal hygeine to you, which is horrible to have to experience, or have terrible personal habits.
The most important thing when touring is to be mentally strong and keep as positive an outlook as possible on absolutely every challenge you meet. You also need to care for the well being of your touring buddy as well as yourself.