I think only the people who truly understand depression can help the depressed, and they can only do that when they aren't stuck in it themselves.
Being happy is something that just comes and goes on its own, and no one can really control it. If you grasp at it, it goes away, and if you just appreciate it while it is there, it stays a while longer. When it goes, if you don't freak out, it comes back sooner. There are things that encourage happiness to come, like doing things you find relaxing, doing things for others (even little things like feeding the pigeons), thinking of things you are grateful for like the sun.
If someone is depressed and judging you for being happy, they are just cutting themselves off from your happiness. It isn't a reflection on you for not joining them in their depression. If they happened to talk with you, they would see that you understand their depression. Since you can't control others, you can't live your life depressed so that other depressed people don't judge your for being unlike them. People stick other people in categories without getting to know them and you have zero control of what other people think of you and what category they stick you in. Instead you could be concentrating on what YOU want from your life. Taking care of yourself is the best thing you can do for humanity. Your energy will help others who are open to other people's energy to have a better day, and when you do talk with depressed people who genuinely want to change, you can share what you know. Think about if you had a younger sister who was depressed. What would you want for her life? Do that!