I assume you are a beginner
when you press click button it fires postback event and call Page_Load event, which rebind the repeater and calls Button2_Click therefore no items is checked use the following in page load method.
DataTable dt = new DataTable();
for (int i = 0; i < 10; i++)
dt.Rows.Add("testing" + i.ToString());
Repeater1.DataSource = dt;