Honestly to me it's just raw emotion that goes into a proper apology, unfiltered and pure emotion. Now, I'm not talking about that kak you give your friend when you took their last oreo and that really annoyed them that one time, I'm talking about when you've properly screwed up and betrayed your friend's trust, I'm talking about that time your friend stopped speaking to you forever because you screwed up so solidly that they literally cannot even stand the look of you anymore. I'm talking the kind of apologies we never want to give but always want to receive.
Read that last line again, swallow your pride, pick up the damn phone and just do it. We get that you thought you were right to begin with and then the friend brought it to your attention that you really were just being an asshole and now you feel really stupid but also really shit and you're hoping that it sort of just goes away with the summer, but you and I both know it won't and the next thing you know years have gone past and you haven't spoken to your friend since back when dinosaurs roamed and then it's too late because that friend has realised that they really don't need the energy vampire (yes that's you) in their life and that they are done with you.
So do it today, phone up the long lost loved one and apologize...