Can't restore database?

Im trying to restore, let's say, db1. But some of my colleagues are using db2. We're in the same sql server. Why can't I restore my db1.bak even if Im the only one who's using it...?

I tried "Take offline" then "Bring online" yet no good.

Maybe because we're using the same login credentials like 'sa'...?

PS, I can't turn off sql server because others are using it...

Error message:

Restore of database 'db1' failed.

Exclusive access could not be obtained because the database is in use.

EDIT

I am able to restore db1 when I'm the only 1 who is connected. Literally, I am the only one who is using/connected to the SQL Server.