What are the things a SQL Server Agent won't start?

When I'm trying to start SQL Server Agent, I got this error:TITLE: Microsoft SQL Server Management StudioUnable to start service SQLAGENT$SQL_NAGA on server NAGA. (mscorlib) ADDITIONAL INFORMATION: The SQLAGENT$SQL_NAGA service on NAGA started and then stopped. (ObjectExplorer) Can you guys help me to solve this problem? I'm using 2014 Enterprise Edition.

Using @@version I got this: Microsoft SQL Server 2014 - 12.0.4213.0 (X64) Jun 9 2015 12:06:16 Copyright (c) Microsoft Corporation Enterprise Edition (64-bit) on Windows NT 6.3 (Build 10586: ) I also found this error event related to SQL Server Agent in the Windows Event Log Log Name: Application Source: SQLAgent$SQL_NAGA Date: 1/31/2016 6:17:24 PM Event ID: 103 Task Category: Service Control Level: Error Keywords: Classic User: N/A Computer: Naga Description: SQLServerAgent could not be started (reason: Unable to connect to server 'NAGASQL_NAGA'; SQLServerAgent cannot start).

UPDATE:

Error log of SQL Server Agent

SQL Server and SQL Server Agent both logged on as Local System Account