SQL Server 2008 Login user mapping does not persist

use master;
GO

create login [<domain\user>] from windows;
GO

use [<yourdb>];
GO

create user [<domain\user>] for login [<domain\user>];
GO

ALTER ROLE db_datareader ADD MEMBER [<domain\user>];
GO

It happens sometimes when user has no connect permission or the permission has been revoked.

Grant connect to [user]