Guys, Levi is insecure having never had much security in his life (according to the letter) and not much human intimacy. He never realized how good it would feel to take comfort and joy in the arms of another person. And I'm guessing because him and Erwin didnt start out in a typical relationship where both parties know where they want to go with each other, it left him further muddled in terms of how he felt. Hence, it made sense to him to take the easy way out, write a letter and run.
Guys, Levi is insecure having never had much security in his life (according to the letter) and not much human intimacy. He never realized how good it would feel to take comfort and joy in the arms of another person. And I'm guessing because him and Erwin didnt start out in a typical relationship where both parties know where they want to go with each other, it left him further muddled in terms of how he felt. Hence, it made sense to him to take the easy way out, write a letter and run.