Each other person shows not only their opinion of me, but also my opinion of them. I know that their opinion of me is important in all kinds of ways, but what is my opinion of them for?
For example, my wife likes me well enough, but I don't like her much:
Does that impact the chance of having children, maybe? If so, it doesn't seem like it has much of an impact, since we've already had two children after not many years.
And as far as I know there's no way to switch your character aside from death*, so it's not as if the opinion your current character has will matter after you take control of your heir. I suppose it can indicate incompatibilities between me and other people that might flag problems I should deal with. Does it matter otherwise?
* During normal play, I mean. I know I can save and load as someone else, but the sort-of cheat of switching to NPCs to manipulate the state of a savegame doesn't interest me. I'm only interested in the opinion's relevance, if any, to actual game mechanics.
Best Answer
It has an impact when the person is your liege. Negative opinion means your liege gets less taxes out of your demesne and less troops if (s)he should decide to raise your levies. Other than that, the opinion modifier is mostly used by the AI.
Also, from what I understand, your opinion of your spouse does not impact fertility.
(As an aside, your own opinion is a handy indicator of a vassal being in a faction conspiring against you. Mouse over your opinion of a character and if you see a -10 modifier "Faction member" you know they are up to no good. At this point use standard tricks to get them out of the faction before it becomes too powerful.)