Timer.start() unexpectedly?

private void PlayerClockInPanelControl_Load(object sender, EventArgs e)
{
    InitializedMouseDownEvent();
}
private void InitializedMouseDownEvent() 
    {
        aTimer = new System.Timers.Timer();
        aTimer.Elapsed += new ElapsedEventHandler(OnTimedEvent);
        aTimer.Interval = 2000;
        aTimer.Enabled = true;
    }
private void OnTimedEvent(object source, ElapsedEventArgs e)
    {
        this.ShowErrorMessage("Hello");
        aTimer.Stop();
    }
private void picturebox_MouseDown(object sender, EventArgs e)
    {
        aTimer.Start();
    }

private void picturebox_MouseUp(object sender, EventArgs e)
    {
        aTimer.Stop();
    }

我的想法是，计时器应该只在MouseDown事件被触发时启动，但现在一旦表单加载，它每秒显示"Hello"。正如代码中提到的，在MouseDown事件期间只有aTimer.Start()被调用。我可以知道怎么了吗?