Listbox Items Question

I'm sending Selected items form a listbox to a textbox by pressing a button using this syntax:

private void btnDesserts_Click(object sender, EventArgs e)
        {
            textBox1.Text += lbxDesserts.SelectedItem.ToString() + ", ";
        }

but when i click the button only 1 item is inserted into the texbox, but i selected 3 items.

Any idea?