When thinking about the world and the universe, many questions about how we fit in and how we should look at our lives start to spring up:
You'll notice that these questions can be used to justify either opinion: you could argue that the universe is large->nothing I do matters->so who cares about life, or you could argue that the universe is large->my problems aren't so big->don't sweat the small stuff. Your outlook on life is really determining the conclusions you draw from information.
That last question about why/how you should/can be happy when other people are miserable is a particularly interesting one because it can create a sense of hopelessness and meaninglessness when you think about the amount of pain, suffering, and death going on all around is in the aggregate. It can feel dreadful and overwhelming.
Now, despite how terrible of a thought that is, no one is really asking how to live happily knowing that people are dying—the answer to that is simple: focus on the good things in your life. If your parent/spouse/sibling/child is upset for no apparent reason, you can either go and be upset with them or focus on your own life and control your own emotions. Everyone knows that, in any given example, if someone else is unhappy, you don't have to be, too. Many times, people are only suffering because of the thoughts that they are thinking and not from anything real, so you'd be miserable for no reason if you joined them. But even when they are, such as in the instance of losing a loved one, there are often times when you can't do anything about it, anyway. So now you've got two situations where it's perfectly rational to focus on the good and let people be miserable because it's either their own doing or there's nothing that anyone can do.
So, you know how to focus on your own growth and a couple of examples that prove when and why you should focus on yourself even when others are unhappy. However, you're probably still unconvinced, and that means that there are really two questions behind this question:
These questions are often asked because people are tempted to join in on what everyone’s talking about (and it’s almost impossible to avoid the news and other people’s talking about it with something like a pandemic) and tempted (due to humans’ natural ability to empathize with other humans) to join in on the emotions that other people are sharing. Inspirational messages like, “Everyone—including those people—want to be happy, so you owe it to them to try,” (though true) don't always address the nagging feeling that guilty people have, so let's go through the faulty logic is in thinking that everyone should be upset just because someone else is:
So, anyone could give you a speech about how you need to live your life happily and to the fullest because those people who are suffering want others to and wish they could be doing that as well, but you’ll just be thinking, “But I can’t ignore their suffering!” But the above list—the list showing how illogical it is to feel obligated to feel bad when others feel bad—shows that it doesn’t do anything and doesn’t help anyone. Instead, you only have two options to really either help those people or make the sacrifice worth it: help the people in need with a positive attitude to make their lives better while going through this time, or be happy and live your life the best you can so that you’re not a burden on anyone else or don’t bring others down (making them worse off).
And, let’s not forget, if you feel sad and mopey around someone who really has something to be sad and mopey about, those people usually resent you: “I have _________; what’s your excuse?” Live your life well and happy for the people who are not; they want you to…and they definitely don’t want you to be mopey and sulking around them when you don’t have any reason to be.