c#鼠标事件序列

问题为什么此代码只生成一个消息框“Down”?我没有起床。如果我屏蔽了代码,上行就正常了。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Mouse
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }   
        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
           MessageBox.Show("Up");                     
        }
        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
           MessageBox.Show("Down");
        }     
    }
}