在方法内定义的变量为什么不会丢失数据？该怎么处理

using System;
using System.IO;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args) {

            //总行数
            int totalLineCount = 0;
            //目录
            string directory;

            if (args.Length > 0) {
                directory = args[0];
            }
            else {
                directory = Directory.GetCurrentDirectory();
            }
            totalLineCount = DirectoryCountLines(directory);
            Console.WriteLine(totalLineCount);
            Console.ReadLine();
        }
        /// <summary>
        /// 计算目录数
        /// </summary>
        /// <param name="directory"></param>
        /// <returns></returns>
        static int DirectoryCountLines(string directory) {
            //这里的变量不是每次方法运行一次就会把次赋为0吗？为什么还可以累加上次的结果。
            int lineCount = 0;
            foreach (string file in Directory.GetFiles(directory, "*.txt")) {
                lineCount += CountLines(file);
            }

            //递归子目录
            foreach (string subdirectory in Directory.GetDirectories(directory)) {
                lineCount += DirectoryCountLines(subdirectory);
            }

            return lineCount;
        }

        /// <summary>
        /// 计算行数
        /// </summary>
        /// <param name="file"></param>
        /// <returns></returns>
        private static int CountLines(string file) {
            string line;
            int lineCount = 0;
            FileStream stream = new FileStream(file, FileMode.Open);
            StreamReader reader = new StreamReader(stream);
            line = reader.ReadLine();

            while (line != null) {
                if (line.Trim() != "") {
                    lineCount++;
                }
                line = reader.ReadLine();
            }
            reader.Close();
            return lineCount;
        }       
    }
}