I'm sitting here being sad and depressed at almost 23 years old when I could've died when I was four.. I remember my fourth birthday, I got a power ranger cake, my whole family was there and I was as happy as can be. This kid probably spent it in the hospital, suffering. I feel like an asshole..
Oh, you're not an asshole! You can't take on responsibility for everything bad that happens in this world, you just have to take care of yourself and be happy. This boy probably also got cake and presents and had a great day, despite his illness. You just have to find something worth living for, and I strongly recommend crafting (crocheting/knitting). Just grab some cheap yarn and a hook, google some easy patterns and you're good to go! You'll be able to make lots of nice stuff, and if you want to do good, there are several projects going to crochet/knit stuff for sick children. I do it all the time, often while watching TV or on the bus, and I never feel as calm and happy as in those moments.
And it's a four year old.