Patrick disappeared after a long bout of abusing drugs and generally screwing the people in his life over. He ditched out of their apartment, leaving Peter to juggle the rent. In contrast, Michelle left without saying goodbye, but it was because she was struggling with Wiley's death and her own guilt. She hurt Peter emotionally, but even still, he never stopped loving her.
Agreed with Gus, also Patrick gave the same self-serving, mollycoddling justifications for his disappearance as he always did for anything else he did wrong/ that went wrong for him. Maybe he's living a better life now, but he's still evidently much the same "I need to take care of poor little me" guy.