psql: error: connection to server on socket "/var/run/postgresql/.s.PGSQL.5432" failed: FATAL: Peer authentication failed for user "postgres" (Ubuntu)

When I try to open psql with this command:

psql -U postgres

I get this error:

psql: error: connection to server on socket "/var/run/postgresql/.s.PGSQL.5432" failed: FATAL:  Peer authentication failed for user "postgres"

But it connects successfully when I use:

sudo -u postgres psql

Can someone please explain what is happening and how to diagnose/fix this problem? My pg_hba.conf contains the following:

# Database administrative login by Unix domain socketlocal   all             postgres                                peer# TYPE  DATABASE        USER            ADDRESS                 METHOD# "local" is for Unix domain socket connections onlylocal   all             all                                     md5# IPv4 local connections:host    all             all             127.0.0.1/32            scram-sha-256# IPv6 local connections:host    all             all             ::1/128                 scram-sha-256# Allow replication connections from localhost, by a user with the# replication privilege.local   replication     all                                     peer