We set pattern lock on mobile to prevent phone data from other persons. if any other person or any family member try to unlock pattern lock without knowing the pattern then after some wrong entries mobile asks google account to reset password. If you have forgot google account too then you can remove pattern lock by hard reset method.
1.Power off your phone
2.Press together volume up + home button + power button
3.Release your fingers of the phone when Samsung logo appear
4.Choose with volume buttons "wipe data/factory reset" and select it with power button
5.Choose "yes" with volume buttons and select it with power button
7.Wait until the phone restarts