At thy right hand there are pleasures for evermore (Ps. 16: 11)
A brother offended is harder to be won than a strong city: And their contentions are like the bars of a castle.Proverbs 18:19
If you cross someone, or let them down, and they were a comparative stranger to you, there might be some level of disappointment—but it is not likely to crush you. This is the case even if the offense were maybe above average. But when someone close to you does it, the event is felt as more of a betrayal than anything else. And a betrayal cuts deeper than anything.
In this proverb we are not told if the offended brother is in the right with regard to the offense. If he was, and decides that he cannot trust you, he is simply being prudent if he moves out of range. You are not trustworthy, and so he will retreat behind strong city walls. And in such a case, it is likely that all the siege engines that you bring to bear are only going to make things worse.
But it is also possible that he is offended, and there are no grounds for the offense. You had a strong relationship, and all of a sudden (it seems) something went south. He took offense when there was no offense given. When this happens, you still have the problem described in the proverb—he has still retreated into his fortress. But now he is not in there defending himself, but rather is in there attacking and wronging you. Taking offense where none was given is a kind of passive/aggressive slander. The person does not accuse you of anything, but is acting as though he could if he wanted to.
When inexplicable estrangements like this happen, the best place in Scripture to go in order to understand it would be James 4. Where do quarrels among you come from? If a chapter begins that way, then perhaps the answer is found within that chapter.
And the answer is that in such cases, envy and jealous ambition is the culprit. And in the first scenario, it is likely the culprit as well, only this time it is outside the castle.