Microsoft has stated, "If you continue to use Windows 7 after support ends, your computer will still work but it might become more vulnerable to security risks and viruses."* The reason is that Microsoft is no longer providing technical assistance for users of Windows 7. Microsoft will stop producing any operating system updates required to close any vulnerabilities that are found in Windows 7. Depending on the nature of the threats involved, Windows 7 may be susceptible to viruses and risks to which more recent iterations of Windows are immune. Due to this, we strongly recommend that you upgrade your Windows 7 computer to a supported operating system as soon as possible.
Here is the Microsoft announcement about the end of support for Windows 7.