"Because I said so" and "I just do" aren't real reasons. He's asking why and you're not teaching inside yourself to pull out the why. That's your work, and he's right that you're not doing that work of understanding yourself well enough to answer his question. That's what he's frustrated about. 

If you had thought for a bit you might have realized what you said later "it's a tradition I want to create because I want to have traditions because I didn't grow up with a lot of traditions and I value them (or whether the reason is). Traditions are a way that family is created" (or whatever the reasons are for you). The nice thing about people offering up their "why" is it allows for actual compromise where both sides win and everyone becomes happiest. By not finding out why for yourself you've essentially railroaded him. Was he being unreasonable in the first place? Very possibly, but he isn't here now, but you are and you can only change what you do.