Magnus has insecurities that have lasted centuries. Alec repeatedly ignoring him and denying they are together in his effort to hide his sexuality make them 100x worse. Especially since Magnus think's Alec is in love with Jace & starts believing he's just a warm body for Alec to use because he couldn't have Jace. Magnus ANGST. Oblivious Alec. Not blind or stupid J-C-I-S.