It was emotional pain
At first, I had friends. Very loyal friends.. but. ALL OF THEM, all of them were killed. Right there I made up my mind, I would destroy every single one of those red shapes. As I played I killed them and made new friends. But my new friends, they died too. I was feeling desperate, I started thinking I was in an infinite loop of losing my friends, making new friends and most painful of them all, losing them again. I stopped making friends. But I was alone. I felt sad. Then I realized I had 50 energy things. I made a lot of friends. An army of friends. But that boss. It had a shotgun. It killed my friends. No. I couldn't take it. I killed the boss. And continued playing. Until wave 25. There was another boss. I was alone and feeling sad. I was killing that boss too. Right when I was killing it, I realized. That boss, it HAD FRIENDS. I killed the boss. I killed it's friends, was I the bad guy. Was I the world's enemy. I made up my mind. I would save the world. By killing the biggest thread, myself. But there weren't any enemies. They weren't coming. And then I remembered. I remembered my first friends. The vow I made back then. TO KILL ALL OF THEM. I had done it. But I couldn't save the world. But, but do I need to save the world, No I wasn't trying to save the world from the beginning, I was trying to save my friends. Then I realized, I can have friends. I CAN. There aren't any enemies left. I made 2 friends. I only had 14 bars. So now I have 2 friends. 8 and 6. We will live together. FOR ETERNITY.