This is a question many historians and sociologists spent their time studying and there surely are many factors involved. I can only give you the answer of what I have experienced myself/theorized. We don't "hate patriotism" as a concept but most people in day to day Germany become uncomfortable with patriotism , suspecting the other person may be right wing (neo nazis), ect. This of course only applies to german patriotism, most Germans are perfectly okay with people of other nationalities being openly proud of their country, even if it might seem a little weird to us because we just don't have that sentiment here. Past patriotism (of course there were many other factors) has led to genocide and war so we don't trust patriotism because it could easily escalate into radical ideas/romanticize WW1 and 2. There is this quote that really captures the situation going "healthy patriotism sounds a lot like benign tumor".