okay, major spoiler (well... obviously) and it involes new characters so anda should atleast find out who they are atau it wont make much sense.
Alois made a contract with Hannah. What he wanted was for no one to have Ciel's soul. So, Sebastian and Claude had to battle thinking that whoever won would get his soul. What they didn't know was that Hannah turned Ciel into a demon. Now Ciel has no soul and earlier Ciel ordered Sebastian that until he ate his soul he remains his butler . Since he can't Sebastian is Ciel's butler forever.
In short,(tho my first keterangan was quite short) Hannah turned Ciel into a demon Sebastian has to serve him forever since he can't take his soul.