Cultural appropriation is something that a lot of people have a hard time understanding. Many can only understand some instances of cultural appropriation, but not others. We as a society can't pick and choose instances where we think it is right, and times where we think it is wrong based on what we agree with. Any instance that offends the culture you are claiming to be "appreciating" is not right.
Cultural appropriation is defined as, "When a person takes something with cultural significance from another culture for their own, but doesn't respect or understand the cultural significance that something has. Instead they only claim to 'like' the culture because of what they can gain from it." It's important to understand the context behind why people get offended by this.
Cultural appropriation is not cultural appreciation.
When you're culturally appreciating a culture or an aspect of a culture, you'd go to a friend who is a different culture's wedding and wear their traditional dress.
You'll never be in their shoes.
Unless you are a part of the culture, you will never have a full understanding of it unless you're around the people who are. Society can't claim to understand why a culture is upset by appropriation if they're not willing to have a conversation with the people they are representing.
A culture should not be a mascot.
This is one of the most difficult aspects for people to understand. The most popular culture-to-mascot example is between Native Americans and sports teams. Terms like "redsk*n" have a background definition of the time when colonies, state governments and companies paid white people to kill Native Americans and used their scalps or even genitalia, as proof of their “Indian kill.” Many people are supporting the campaign #NotYourMascot to change this aspect because people from the actual culture do not approve of their culture being mascots.
It shouldn't be just for fun.
If the sole aspect to borrowing something from another culture is because, "it's just a costume" or "it's sexy," chances are you are appropriating and don't need to be wearing the cultural aspect. There are real people that have these aspects of their culture for specific reasons and not for Halloween costumes.
Why it's a problem.
It may not be intentional, but people view those who appropriate in a better light than the people whose culture they borrowed. The reason people are getting so upset with appropriation is because the people who are going around and wearing their own culture are seen as lazy or other negative stereotypes, while the people who are appropriating are seen as being cutting edge and viewed in a positive light. It's hard to not get upset when people are getting praised for your culture, while the culture it belongs to is being ridiculed.
So, the next time you're deciding to borrow an aspect of another person's culture, it's not a bad idea to ask yourself if it's the right thing to do. Have a conversation with people in the culture and see if you may or may not be going too far. It only hurts to assume, but never to ask questions.