The Chekov and Irina situation isn't remotely similar to the others. My impression is that she left him, so that she could follow her own path. A path that didn't include Chekov.
This is true, of course. I was speaking of not the specific circumstances of each character, but of the on-screen dynamics, i.e. "You left without saying goodbye.", "You had your world, I had mine.", etc., the idea being that a past relationship was at the time irreconcilable, and lingering feelings remained.
As I get older, I become more convinced that all relationships are basically the same. Whether a character (or real person) is devoted to church, music or motorcycles, the basic conflicts are the same in their respective conflicts.
Having been both the dump-er and the dump-ee, all I know is that my relationships went *poof*, and the end result was the same regardless of who instigated it!
With Chekov and Irina, it didn't work out. With Decker and Ilia, it didn't work out. That's really all I was driving at!