Any way to connect postgres through username

Hello,

I am very new to PostgreSQL. I am trying to connect to database via a different user which is created by me.

psql -U user1
but this gives me error message saying database user1 does not exist.

When I try psql -d mydb -U user1 this works fine.

Can any body suggest me if there is any way to connect to database through psql command line without using the -d database parameter.