On other days, Z provoked X, X get upset and whine, or X hit Z and Z cry. The permutations were limitless.
As X grew older, he was more demanding and fixated on how things were run. He was obstinate and really annoyingly persistent. That did not work well with Z. Z teased X and baited him over really lame stuffs.
At the end of the day, when they played well together or displayed affection for each other, it never failed to melt my heart and wished they stayed that way forever.