check below sample code,
<head runat="server">
<title></title>
<script src="Scripts/jquery-1.8.0.min.js"></script>
<script src="Scripts/bootstrap-select.min.js"></script>
<link href="Content/bootstrap-select.min.css" rel="stylesheet" />
<script>
$(document).ready(function () {
$('#DropDownList2').click(function () {
var str = "";
$("#DropDownList2 :selected").each(function () {
str += $(this).text() + " ";
});
alert(str);
});
});
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:DropDownList ID="DropDownList2" CssClass="selectpicker" runat="server" multiple="multiple">
<asp:ListItem>Allahabad</asp:ListItem>
<asp:ListItem>Kanpur</asp:ListItem>
<asp:ListItem>Rewa</asp:ListItem>
<asp:ListItem>Bhopal</asp:ListItem>
<asp:ListItem>Indore</asp:ListItem>
<asp:ListItem>Jabalpur</asp:ListItem>
</asp:DropDownList>
</div>
</form>
</body>