They only know the old you
With some friends, you feel like they really don’t know who you are today. When you talk, you only talk about the good old days. You revert back to the state of mind or maturity level you were at when your friendship first formed. You may not even like who that is today – you don’t like the old you, or that old version of this friend – but you only get along when you revert to that. Ultimately, you feel like you have a second identity that this friend knows nothing about. Having this friend around your closest friends now would be awkward because each party sees you entirely differently.