Why do you use Server side event to get selected item in Textbox.
Use Javascript it will help you to improve the performance.
Here is example
<head runat="server">
<title>Untitled Page</title>
<script language="javascript" type="text/javascript">
function callME()
{
alert(document.getElementById("lst1").value);
}
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:ListBox runat="server" ID="lst1" onclick="return callME()">
<asp:ListItem Text="a" Value="a1"></asp:ListItem>
<asp:ListItem Text="b" Value="b1"></asp:ListItem>
</asp:ListBox>
</form>
</body>
Hope it helps