My MacBook Pro keeps freezing and then rebooting

Can you try booting into safe mode and see if it seems to work more normally then?

Also, if you get into safe mode, create a new temporary admin account so you can boot normally

into that account and test if the rebooting still happens. This will identify if the problem is system wide

or just one account.

(How to reboot into Safe Mode). This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The safe boot will be slower than in normal boot mode and can take some time which is Normal.  

02) Check to see if the problem is still present while in Safe mode. Sometimes a Safe Boot followed by a Normal Boot, will many times, correct the problem.

03) There might also be a problem in the main User Account causing the issue. Create a temporary Test Account and assign as administrator, to further isolate the problem: 

(How to setup users, guests, and groups on Mac).

Log out of the Main User account and log into the Test account and try to initiate the problem again. If the problem is still present in the Test account - then, the problem appears to be a System Wide issue on the computer. If the problem is no longer affecting you in the Test account, then the problem is associated to a specific user account issue.