Yeah that was the first thing that I learn with my son. Not much with my older. But when I had my son and started to do somethings at a certain time and then didn't do it he would be upset. Then I had my youngest and really got in to a rutine. They like to be in bed by 6-7pm. They know that when they get up we go to the kitchen to make breakfast every single day and my son sits in the tabble and my youngest goes in to her high chair and they wait for breakfast. 

Quoting Shortfuse007:

Kids get so used to routine, but it gets disrupted, they get upset. Both of my girls are that way.

Quoting loren2012:

I have that problem on sunday my husband makes them breakfast but when they want milk  should  be in the cup that i buy for them  for my son is a cup of spider man and for my daugther is tinker bell ,one day my  other son drink water on my little boy cup and he get very angry hahaha since that day  no one drinks  on those cups.my husband just  laughs.

Honestly, the only things I will let them "not share" are any toys that were given specifically for them. When it comes to dishes or anything else it's fair game. Now if I had a personalized cup/dish etc for them then I'd be sure that only they would be using it.