Why is my laptop constantly restarting?

Check your laptop's cooling mechanism.
Make sure that the vents and fans have adequate air flow, and that no dust has built up inside the computer over time.
Check for external hardware tampering.
If your laptop is being restarted without any clear reason, then someone may be physically tampering with your laptop in some way.
Check your device's power source.
If the power adapter is not providing a stable power supply to your device, your laptop may be restarting due to insufficient power.
Check for any newly-installed software.
If you have recently installed any new software, it may be conflicting with existing software. Check to see if you can uninstall the newly-installed software, and then, see if the issue gets resolved.
Check for virus and malware.
High or low-level virus or malware can possibly cause problems like a laptop restarting with no warning.
Check any recent updates.
If you had recently updated any of your software, then a reboot may have been necessary. Check to make sure that the updates have finished successfully.