C# timer starts automatically

I don't know why my timer starts automatically. Maybe anybody can help?

public int sek;
void TextBox1TextChanged(object sender, EventArgs e)
{
    sek = sek+Convert.ToInt32(textBox1.Text);           
}

void Button1Click(object sender, EventArgs e)
{
    label2.Text = Convert.ToString(sek);
    timer1.Start();
}

void Label2Click(object sender, EventArgs e)
{

}

void Timer1Tick(object sender, EventArgs e)
{
    label2.Text = Convert.ToString(sek);
    if (sek==0){
        Console.Beep(1000 ,500);
        MessageBox.Show("Times is out");
        timer1.Stop();
    }           
}