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Apologize early and often; admit that you were wrong (if you were) and take responsibility.  Tell him that you were wrong and that you won’t do it (whatever it was) again. And then don’t do that thing again. Don’t use the sorry-not-really-sorry line of “I’m sorry you got so upset.” It shifts the responsibility onto him and makes it sound as if you’re not sorry for your actions, you just wish he wouldn’t get mad about it. If he responds with a--potentially justified--angry text, apologize again. Continue apologizing if he comes at you with more angry responses. Simply say something like, “I’m sorry--I was wrong.” Just apologizing or trying to explain that your intentions were good won’t work.  Saying you’re sorry isn’t enough--you need to show you recognize the negative consequences your actions had on him and that you sincerely regret it.  If he feels like you genuinely understand why your actions made him mad, he’s more likely to feel validated and begin forgiving you. Even if you don’t necessarily think his feelings or reaction is justified, apologize anyway. If you want him to like you again, you want him to see you understand how he’s feeling. Even if you say you’re sorry, it won’t be enough to make him like you again if you then start saying things that will escalate the situation.  Don’t, for example, say anything about thinking his reactions were irrational or unjustified. It will make him feel like you aren’t really sorry and don’t really understand the situation--and he’ll go back to being mad.  Don’t bring up things he’s done to upset you in the past. Turning the conversation into a tit-for-tat about who did what to who isn’t going to diffuse the situation. It will only make it drag on longer and make him less likely to forgive you. Asking for his input on what you can do shows that you’re listening to him and genuinely want to know what would make things better from his perspective. For example, you might say something like, “I know you had to wait 45 minutes for me and that it probably felt like I wasn’t making you a priority. How can I make it up to you in the future?” Humor is disarming. If you can make him laugh, or even smile a bit, you can get your foot in the metaphorical door.  Try being humorously self-deprecating. If humor is disarming, affable self-deprecation is doubly so. So try poking a little fun at yourself or acknowledging one of your endearing flaws. You might text him something purposefully ridiculous, like, “I’m really sorry I was late picking you up--we both know I’m a klutz, and I must have walked into at least five walls trying to get here.” Or you could text something more honest but still gently self-deprecating, like, “You know how I look at the time on the clock as a challenge instead of a warning? Well...the clock won.” Particularly if the guy’s mad because he feels like you ignored him or his needs in some way, remind him that you do think about him--often. For example, you might text him with an affectionate reference to something you saw that reminded you of him (bonus if it relates to a private joke between you), like, “I just saw a car with a Florida license plate and it reminded me of all the stories you tell about growing up there. It made me smile.”

Summary:
Apologize. Show that you understand the impact your actions had on him. Avoid escalating the situation. Ask what you can do to make it up to him. Make him smile. Let him know you’re thinking about him.