While I agree that yes, an obese person needs to lose weight, telling them that and offering no other help is like telling someone with depression "why don't you just cheer up?" It's totally unuseful advice that incorporate all sorts of gross value judgments about the person. It doesn't solve problems.
I've been overweight - not obese, but close at times - all my life. I'm now on a weight loss program and seeing significant success. It took years of mental and physical issues to get me to that point and it is a struggle every day. It's hard work. That's why people are proud of losing weight: it's an achievement. It represents a permanent change to lifestyle and behavior that takes weeks, months, even years to effect.
Please don't discount people's experiences by saying things like "oh, just lose weight." Consider the human being in there, and how to reach them. Don't assume that because it's easy for you to be fit and skinny, it's easy for everyone.