When apologising to someone, do you give a reason for why you did something, or do you leave the reason out?

I want to apologise to my nephew after I revealed he has a different dad to his siblings. However, I don't know if I should explain the reason for why I did what I did or if I should apologise and let him know how sorry I am and how deeply I regret it, but leave the reason out.

When apologising to someone, do you give a reason for why you did something, or do you leave the reason out?
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