Can’t connect my watch to my Phone

I’m trying to connect my Iwatch to my IPhone. It was connecting well and then it stopped and asked for my Apple ID and Password. So I entered in my current Apple ID and Password that is also set up to my phone and it denied it. Apparently it needs a old iCloud email of mine? Well I located the email for my old iCloud account but I can’t go through still because I don’t know the password. I need help. Why can’t I simply be able to use my current Apple ID and Password that’s used on my phone and just be done with it?