Maybe it's the definition of terms used. Fantasy can be Star Trek or My Little Pony. Similar, but no where near the same.
You go along and you want to believe that you are both seeing the same things. That you are both getting the same experience from the journey. But you're not. To one, this has always been one of the strengths. Two sides of every story. It's only now and then, that you notice differences in perspective. Interesting observations you hadn't noticed. A view from which you had not been privileged to before.
Then one of you says, "You know, maybe I don't ever want to watch My Little Pony again. Discuss...".
At first, this seems to come from no where. Then you give it some thought. They must have been thinking about this for a while. You don't just wake up one day and say no more MLP. People think about things. They mull them over. They talk to strangers on benches about how much they want to leave MLP behind and run away. Leave all their troubles and worries behind. It's only then do they mention it to their other.
There have been signs that MLP was a strain on the journey. Communication has always been a weakness for them. Things get looked over. Ones focal point is a little different than the other.
The hurt and painful parts of the journey, you over looked. For better, or worse. No matter how much you were hurt, you took that pain and stuffed it deep in the back of your darkest grey matter and you forget it. Because that is what you do. The joys of the journey mean infinitely more than the momentary pain. That is why you have one an other. To be there for the other. And to have someone when you need help.
Which is great...for one. The other though? They have a different perspective on things. To the other, maybe the journey is about something else. Maybe to the other it was about more than just the other. It's such a little thing to one and such a large thing to the other.
Two people looking at the same thing. One sees a boulder, the other sees a marble.
So now it's out there. One of you is tired of My Little Pony. Tired to the point that they would rather journey in a different direction. It annoys them that you always see marbles when there is clearly a boulder in front of you.
You see, the burden is directly proportional to the point of view. They look at the same thing. One sees a marble, they pick it up and they play. The other sees a boulder and tries to carry it on their own which is very difficult and causes undue stress, strain and worry. Not seeing it doesn't make it any less real. It's real to the one.
The one with the marble doesn't understand why the other won't just set the boulder down. The two of them together could figure out what to do with the boulder. The one with the boulder doesn't understand why the other is incapable of helping and would rather play with marbles while they carry the boulder alone.
Worlds apart, separated by a point of view.