He did it in the first movie, whenever he tinkered. The same goes for the second.
But in Vol. 1, the songs he sang were unrecognizable. They had a sway to them, and maybe sounded akin to classical music. Perhaps the only music he’d ever been exposed to was when he was stuck in a lab. The scientists would listen to classical music while they worked.
Or maybe he heard some tunes while at a dingy bar – alone and bitter. Even when Groot showed up, the hole in his heart was never fully filled. Groot was all he had.
But in Vol. 2, he’s gained a family. He’s surrounded by people that are willing to put up with him, and although he’d never call it love, he knows that he cares deeply for them and they do, too. The music he hums comes from Peter – from better times, where he knows he’s not alone.
You shot up in bed sweating. Peter felt you and woke up immediately too. “Whoa, whoa, what’s wrong? Are you okay?” he questioned. “Take deep breaths, oh my god, what happened?” You did as he advised and took deep breaths. “I had a nightmare,” you uttered. “Aw man, well just lay back down and try to forget about it,” he suggested.
“I can’t, it’s all that’s running through my mind,” you said to him.
Peter then put both of his hands on your sides and went back down to the bed with you. He kept one of his arms draped around you and began to sing the lyrics to Moorage Daydream.
“Keep your electric eye on me, babe. Put your ray gun to my head. Press your space face close to mine, love. Freak out to a moorage daydream, oh yeah,” Peter sang softly to you helping you fall back asleep.