There could be several reasons why your MacBook is shutting down by itself, and it's hard to pinpoint the exact cause without further information or diagnosis. However, here are a few common causes and solutions you can try:
1. Overheating: If your MacBook is overheating, it may automatically shut down to prevent damage to the internal components. Make sure your MacBook is not placed on a soft surface like a bed or cushion, as it can block the ventilation and cause overheating.
2. Battery issues: If your MacBook's battery is faulty or not functioning correctly, it may cause unexpected shutdowns. You can check your MacBook's battery health by clicking on the Apple menu > About This Mac > System Report > Power. If the battery condition is "Replace Soon" or "Service Battery," you may need to get it replaced.
3. Software issues: Sometimes, software issues like corrupt system files or incompatible applications can cause your MacBook to shut down unexpectedly. You can try booting your MacBook in Safe Mode by pressing and holding the Shift key during startup. This will disable all third-party extensions and start your MacBook in a clean state. You can then troubleshoot the software issues from there.
If none of these solutions work, you may need to take your MacBook to an Apple Store or an authorized service provider for further diagnosis and repair.