BOL says:
The dbo is a user that has implied permissions to perform all activities in
the database. Any member of the sysadmin fixed server role who uses a
database is mapped to the special user inside each database called dbo.
Also, any object created by any member of the sysadmin fixed server role
belongs to dbo automatically.

For example, if user Andrew is a member of the sysadmin fixed server role
and creates a table T1, T1 belongs to dbo and is qualified as dbo.T1, not as
Andrew.T1. Conversely, if Andrew is not a member of the sysadmin fixed
server role but is a member only of the db_owner fixed database role and
creates a table T1, T1 belongs to Andrew and is qualified as Andrew.T1. The
table belongs to Andrew because he did not qualify the table as dbo.T1.

The dbo user cannot be deleted and is always present in every database.

Only objects created by members of the sysadmin fixed server role (or by the
dbo user) belong to dbo. Objects created by any other user who is not also a
member of the sysadmin fixed server role (including members of the db_owner
fixed database role):

 a.. Belong to the user creating the object, not dbo.

 b.. Are qualified with the name of the user who created the object.