The answer to 'why is my supply line' is that a supply line issue is typically caused by a clogged or damaged pipe, a leaky connection, or a problem with the shutoff valve. Common symptoms include low water pressure, dripping, or water pooling around the supply line. Solutions can range from tightening a connection to replacing the entire supply line, depending on the severity of the issue.
If the supply line is just leaking slightly, you may be able to fix it yourself by tightening the connections or replacing a washer. However, if the line is severely damaged or the issue is with the shutoff valve, it's best to call a professional plumber to properly diagnose and repair the problem. Attempting complex plumbing work without the right skills and tools can lead to further damage or even flooding.